Background
Dermod, who was born in October 1594, was the son of Murrough O'Brien, 4th Baron Inchiquin and his wife Mabel, daughter of Christopher Nugent, 5th Baron Delvin. Dermod O'Brien married Ellen, eldest daughter of Sir Edmund FitzJohn FitzGerald of Cloyne and Ballymaloe House in County Cork, from that powerful Hiberno-Norman family.