Complex sentence: He was born in County Meath, the third of seven sons of Sir Christopher Plunket, who married Janet Cusack, the heiress of Killeen Castle, Dunsany, was created 1st Baron Killeen about 1426, and founded a prominent Anglo-Irish dynasty.
Split into simpler sentences: He was born in County Meath, the third of seven sons of Sir Christopher Plunket, who married Janet Cusack, the heiress of Killeen Castle, Dunsany. Christopher was created 1st Baron Killeen about 1426, and founded a prominent Anglo-Irish dynasty.