Complex sentence: The invading Norman's forced them north to the River Lee where a territory became known as the land of the O'Leary's, and at Carrignacurra, Carrignageelagh and at Dromcarra, they built three castles.
Split into simpler sentences: The invading Norman's forced them north to the River Lee where they settled on a territory known as the land of the O'Leary's. At Carrignacurra, Carrignageelagh and at Dromcarra, they built three castles.