Answer
John A. Costello served as Taoiseach, or Prime Minister, of Ireland when William T. Cosgrave held the position of Parliamentary Secretary. Cosgrave, a prominent figure in Irish politics, held various cabinet positions throughout his career, including Minister for External Affairs and Minister for Finance. His tenure as Parliamentary Secretary, a role responsible for assisting the Taoiseach in parliamentary matters, coincided with Costello's leadership of the country.