Complex sentence: Art Rooney was the founder and owner of the team until his death in 1988; ownership of the team was then transferred to Art's and Kathleen McNulty Rooney, his wife's, oldest son, Dan Rooney, a former United States Ambassador to Ireland.
Split into simpler sentences: Art Rooney was the founder and owner of the team until his death in 1988. Following his death, ownership of the franchise transferred to Art and his wife Kathleen McNulty Rooney's oldest son, Dan Rooney, a former United States Ambassador to Ireland.