Because Lucky Spencer was a dead ringer for a deceased Irish terrorist named Ronan O'Reilly, he accepted an assignment from Interpol to infiltrate the organization of an infamous crime lord known as the Balkan, whom Ronan had worked for. Lucky was making inquiries at a local Irish pub when a redheaded spitfire named Siobhan accused him of killing her boyfriend. To protect his cover, Lucky told Siobhan he had found God and had reformed. Lulu and Dante, thinking Lucky was in Ireland to track down Luke, followed Lucky there. When Lucky explained his mission to Lulu and Dante, Siobhan overheard, and his cover was blown.
After Lucky was arrested by the Irish police for killing two of the Balkan's henchmen, Luke, dressed as a priest, visited Lucky in jail and advised his son that the best way to neutralize an angry woman was to romance her. Lucky took his Dad's advice and passionately kissed Siobhan. Siobhan reacted by slapping him. The Balkan's henchmen spotted Siobhan hiding and tried to drag her off, but Lucky convinced them that she was his partner in crime. When Lucky was injured in an altercation, Siobhan took care of him, and they grew closer and had sex.
The Balkan ordered "Ronan" to kidnap Brenda Barrett. When Lucky found out that Brenda was in Port Charles, Siobhan asked him to take her with him. Because the Balkan had ordered Siobhan killed, Lucky agreed. Lucky asked Nikolas to let Siobhan stay at Wyndemere while he searched for Brenda. When Siobhan wandered out onto the grounds, she was captured by the Balkan's henchmen. Soon after, Lucky received an email with a photograph of Siobhan tied to a chair. The Balkan tried to make a deal to trade Siobhan for Brenda, and Brenda agreed to be the bait.
Siobhan was freed and, when questioned, she told Lucky that she thought she'd heard one of her captors say the name Jack. Lucky was surprised to find Siobhan packing to go back to Ireland. Siobhan agreed to stay on the condition that she pay her own way, and she got a job waitressing at Jake's. She was determined to maintain her independence and called Lucky on his need to protect her. Both Elizabeth and Lulu were skeptical about Siobhan and Lucky's budding relationship. Edward Quartermaine, however, was charmed when Lucky took Siobhan to Thanksgiving dinner.
When Interpol contacted Lucky with a new assignment as Ronan O'Reilly, Lucky headed for New York City. He was surprised to find that Siobhan had followed him. She convinced him to use her as a decoy so that Lucky could get access to Ronan's safe deposit box. Back in Port Charles, Siobhan enlisted Nikolas' help in trying to get Lucky off the Balkan's case. She also questioned Jax about Jerry Jacks because Lucky thought Jerry might be the Balkan. Coincidentally, Siobhan was on the docks when Jerry fell into the water and seemingly drowned. When Jason questioned Siobhan about the incident, he was suspicious about her version of the events.
Lucky offered to give up a double shift so he could take Siobhan to hear a band and was surprised when she told him she had other plans. He told Siobhan that a while back he had caught her in a lie. He had received a call from her sister Fiona, who was worried because she hadn't heard from Siobhan, when Siobhan had told Lucky she had just talked to her sister. Siobhan wanted to let it go but Lucky wasn't okay with that. Siobhan covered and told Lucky that an old boyfriend had been calling and was determined to see her. She had been trying to discourage him, but if she didn't talk to him that night, she was afraid he might do something desperate. Siobhan told Lucky that the reason she'd lied was because she wanted to keep her old life far away from her new one.
Siobhan went to Diane's office to discuss her immigration status and found Theo Hoffman at Diane's desk. She needed to renew her tourist visa. Theo offered to take her case pro bono, but his probing questions set her on edge. When Lucky arrived at Diane's office, he was surprised to see Siobhan conferring with Theo and questioned her about why she was meeting with Brenda's attorney. Theo explained his expertise in immigration law but said that there was little he could do about Siobhan's tourist visa. He surprised them both when he suggested Siobhan marry Lucky as a way to get her green card. Lucky thought that was not such a bad idea.
The Balkan summoned Siobhan to a confession booth at a church and told her that he was disappointed in her job performance. He complained that the flow of information had been very slow. Siobhan claimed that she had nothing to report because Lucky and his friends hadn't made any progress in finding him. The Balkan wanted to know if they believed Jerry Jacks was the Balkan. Siobhan didn't think so. It was a little too convenient that she gave them the name Jack and then witnessed Jerry's demise. Since then, she'd had to work overtime to keep Lucky's trust. The Balkan warned her if she failed in her mission, the consequences would be dire. To prove his point, the Balkan showed Siobhan a picture of her sister, Fiona, gagged and tied to a chair.
Siobhan tried to end her relationship with Lucky after the Balkan planned to kill him. The Balkan reminded her that the fate of her sister was at stake so she reconciled with Lucky. Although Lucky and Siobhan were back together, a conversation between Jason and Lucky prompted Lucky to reconsider Siobhan's trustworthiness. He recalled the mysterious phone calls and secretive errands Siobhan engaged in and he set a trap for her. Siobhan often asked questions about Brenda Barrett and her upcoming nuptials, so Lucky provided Siobhan with false information about Brenda's wedding. When Siobhan showed up to scout the location for the Balkan, Lucky was waiting for her.
After Siobhan confessed she worked for the Balkan only to gain her sister's freedom, Lucky and Siobhan developed a plan to trick the Balkan. Lucky arrested Siobhan as part of their ruse so the Balkan would not know they were working together. They flew to Ireland to rescue Siobhan's sister where the Balkan's men were waiting. Siobhan pretended she had led Lucky to his demise, but they turned the tables on the Balkan's men and escaped with her sister.
Lucky convinced Siobhan to remain in the United States and they decided to marry so she would no longer face threats of deportation. Lucky remained adamant he had no second thoughts about marrying Siobhan and they made arrangements with a justice of the peace and asked Luke to witness their ceremony. The ceremony was halted was Lucky received a call from Elizabeth. She told Lucky that Jake was in critical condition after being hit by a car.
Jake did not survive the accident. Siobhan supported Lucky through his grief and through the realization that his father had been the driver. She acknowledged the bond between Lucky and Elizabeth and respected their need for time together. Siobhan also agreed to give Lucky space if he needed, but he surprised her with marriage. Siobhan visited Elizabeth to make sure Elizabeth was fine with the wedding. While she was there, Siobhan discovered a paternity test that showed Aiden was Lucky's son. Elizabeth explained she did not want to destroy the bond Nikolas and Lucky had just formed by revealing Aiden's paternity. Siobhan decided it was not her secret to tell and she proceeded to marry Lucky. They were married by a justice of the peace on May 6, 2011, which was Jake's birthday. Lucky had forgotten until Elizabeth showed up on their wedding night to grieve with Lucky over Jake's missed birthday.
After Jake's death, Lucky's relationship with his father continued to be a struggle. Luke left town and claimed he was done with family forever. Lucky acted out by burning down the Spencer home. Siobhan saw the burning house and rushed inside to save Lucky, unaware he was outside. Dante and Lucky heard Siobhan and rescued her.
Siobhan was in critical condition and rushed into surgery. Elizabeth was called in to assist the surgery, unaware Siobhan was the patient. During the surgery, Elizabeth injected Siobhan with the wrong medication which caused her blood pressure to drop. Siobhan survived the surgery and made a full recovery. Lucky was by her side when she awoke and apologized for placing her in danger. He also shared the news that he was Aiden's father. Siobhan admitted she already knew.
Elizabeth visited Siobhan as she recovered and apologized to Siobhan for the error during her surgery. Siobhan was shocked to learn how near death she had been. Siobhan did not believe the medication switch was an error and instead believed Elizabeth tried to kill her so she would have a clear path to Lucky. Siobhan then announced her plans to file a lawsuit against Elizabeth.
A rift formed between Lucky and Siobhan when Lucky defended Elizabeth. He convinced Siobhan to drop the lawsuit, although she continued to doubt Elizabeth's intentions. Siobhan attended Jake's first birthday party at Elizabeth's home and the two women pretended to get along until Lucky was out of earshot. Siobhan complimented Elizabeth on how well she played on role of the grieving mother and used her children to remain close to Lucky. Elizabeth claimed Siobhan would never understand the depth of the pain she and Lucky felt over losing Jake and Siobhan remarked that her grief was a great excuse for anything, including mistakes in the operating room. Elizabeth responded by kicking Siobhan out of her home.
Siobhan became more enraged over Elizabeth's error in the operating room when she learned Elizabeth had been reinstated at the hospital. When she spoke to Lucky about the reinstatement he openly supported his ex-wife and told Siobhan he had taken a case investigating narcotics that were missing from the hospital. Siobhan was disappointed when her husband enlisted help from Elizabeth on his case. He explained that as a hospital employee Elizabeth could alert him of any suspicious activity that occurred there.
Given Lucky's history of addiction and preference for painkillers, Siobhan worried Lucky would not be able to handle working directly with pills. She turned to Elizabeth for help and asked to her to convince Lucky to give up the case. The women disagreed when Elizabeth insisted Lucky was strong enough to remain sober. Siobhan accused Elizabeth of being selfish because the case gave her an excuse to feel important to Lucky. She continued to badger Elizabeth about her choice to aide Lucky in the investigation and taunted Elizabeth by claiming Jake's death was her fault.
Siobhan arrived home and discovered Lucky going through her purse. She immediately assumed he was looking for money to buy drugs. Lucky was hurt that Siobhan did not have enough faith in him to believe he could resist the temptation of the pills and he explained he was placing a new credit card in her wallet. They argued over Siobhan's lack of trust and Lucky stormed off to see Elizabeth while Siobhan experienced pain in her head.
Siobhan received an apologetic message from Lucky and she decided to trust him rather than push him into Elizabeth's arms. Siobhan set out to find Lucky and she overheard a conversation on the pier that linked Anthony Zacchara to the drug ring. When she was unable to locate Lucky she turned to Elizabeth. Siobhan's fears were confirmed when Elizabeth admitted Lucky had been shot up with drugs and she was also trying to find him. They decided to search for Lucky together and Siobhan fell down a flight of stairs as they argued. Elizabeth claimed it was another accident but Siobhan believed otherwise.
Elizabeth convinced Siobhan she needed to be checked out at the hospital and Siobhan agreed to let Elizabeth drive her there. On the way to the hospital Siobhan passed out and Elizabeth took her eyes off the road and crashed into another car. Siobhan was thrown from the car and remained unconscious. She required surgery when she arrived at the hospital. Elizabeth was not in the operating room during the surgery.
Lucky was not present when Siobhan woke up after her surgery but she left him a voicemail in which she apologized for doubting him and professed her love. Siobhan requested to speak with Elizabeth and she thanked her for finding Lucky and saving him although she questioned if Elizabeth had tried to kill her on the stairs. Siobhan asked Elizabeth to find Lucky because she overheard a conversation that might help Lucky with his investigation.
Anthony Zacchara realized Siobhan discovered he was involved in the drug ring and he sneaked into her hospital room and confronted her. She tried to claim she had no knowledge of his involvement but he was not convinced. Anthony dampened a cloth with a chemical and held it over Siobhan's face until he believed she was dead. Elizabeth stopped by Siobhan's room but assumed she was asleep and left. Before Siobhan died, she wrote a message to Lucky about his investigation on her pillow. Lucky arrived to see his wife and was surprised her machines were off. Her body felt cold and he called out for help. Steve Webber tried to revive Siobhan but it was too late. Elizabeth was the last person believed to have seen Siobhan alive and an autopsy was ordered to determine the cause of her death.
After Siobhan's death, Lucky found a letter Siobhan had written him that instructed him to visit a church in Ireland. Lucky complied and he experienced visions of Siobhan. She told him that his son Aiden was in need of immediate medical attention and urged him to return home immediately. The visions continued on the plane trip from Ireland to Port Charles in which Siobhan urged Lucky to have faith and to keep his heart open.