Sean's history with drugs is revealed, as he is known to be a recovering heroin addict which cost him much. Throughout the season, he and Fiona flirt back and forth. However it is shown that flirts slightly with the other co-workers but it may just be how the workers have always been. He also shows a strong interest in her but eventually tells her that he can't be with her. Sean admits that he had killed a man while had heroin in his system.
More of Sean is shown, as he comes into Fiona's life more such as getting her brother Carl off the thug life, at the cost of some of his possessions.
Eventually, they end up falling for each other and in the season finale they almost get married until her father Frank reveals that Sean has started doing heroin again. He gives Fiona the ownership of Patsy's Pies and leaves for good.
In Fuck Paying It Forward, Sean comes back to try to make amends with Fiona by showing up at the house after Liam lets him in. Fiona is angered to see him again. At a diner, Sean states he has been clean for a year, he attempts to pay Fiona back her money for the wedding but she refuses him and reveals she still has affections for him though can't give a him a chance. Fiona receives a shock when Sean is revealed to have gotten married his ex-wife (now wife) again and needs to make amends before starting a life with his wife. Sean apologizes for not telling her and not knowing she still had feelings for him. Fiona gets pissed and throws the money in his face and leaves. At a motel, Fiona soon embarrasses herself after trying to tell his new wife of Sean's past but gets the wrong woman and Sean is left oblivious to her actions.
- The character of Sean Pierce was originally going to be Charlie Peters played by Jeffrey Dean Morgan, but due to filming of Texas Rising, Morgan was only able to appear in one episode, and the character in whole was replaced with Sean Pierce.
- Fiona has dated another man named Sean (mention in Strangers on a Train) who like this Sean had a drug problem.
- He is largely responsible for Frank getting estranged from the family for most of Season 7.