Complex sentence: He was born in the United States of America but spent most of his childhood and adolescence in the Republic of Ireland where he attended Newtown School in Waterford from which he graduated with an honours Leaving Certificate in 1985.
Split into simpler sentences: He was born in the United States of America but spent most of his childhood and adolescence the Republic of Ireland. He attended Newtown School in Waterford from which he graduated with an honours Leaving Certificate in 1985.