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

Pa Mackey is Frank's father. He is a bitter and abusive father and husband. The bitterness apparently dates back to an affair with a young girl when he was a young man that ended in a sort of ostracism from a wealthier segment of the community. The result is that Pa is unable to get a job that he desperately wants and never holds steady work after that. Pa tells Frank that he'll kill himself before he goes to a nursing home.