Who is Seamus Mackey from Faithful Place: A Novel and what is their importance?

Known as Shay, Seamus Mackey is the oldest son of the Mackey family and is Frank's older brother. Shay wants to get away from his family and hates the fact that Frank and Rosie are planning to run away together. Shay's selfishness toward that end prompts him to kill Rosie. Shay even kills his younger brother Kevin when he realizes that Kevin will reveal Shay's role in Rosie's murder. Shay is arrested as the story comes to an end.