Sean Pierce is the former owner of Patsy's Pies. He first met Fiona when a case worker brought her in. Sean helps Gail out when one of her parolees' need a job. He was in love with Fiona Gallagher. He is played by actor, Dermot Mulroney. He is known to be a recovering heroin addict.
In season 5 we find out about Sean's history with drugs. Throughout the season the two of them 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.
In season 6, we see more of Sean as he comes into Fiona's life more. Eventually they end up falling for each other and in the season finale they almost get married until Frank reveals that Sean has started doing heroin again. He gives Fiona the ownership of Patsy's Pies and leaves for good.
In the 8th season, he comes back.