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

Frank's mother is called Ma Mackey. Ma is scarred by a lifetime of abuse and poverty. She demands respect from her children but has none for herself and doesn't understand how to live any way other than by mental and physical abuse. Frank is amazed to see that she's gentler with her grandchildren than she'd ever been with anyone else though she doesn't care if they hear screaming matches among the adults.