Fay, William Patrick Ignatius (1909–69), diplomat, was born 17 April 1909 at Seaview House, Booterstown, Co. Dublin, son of Henry Edward Fay, surveyor at the General Post Office, and Helena Magdalen (neé Browne), and was a nephew of William (qv) and Frank Fay (qv), who were both closely associated with the Abbey Theatre in Dublin. He was educated at Clongowes Wood College, Co. Kildare, King's Inns and UCD, where he graduated BA. Called to the Irish bar in 1931, he practised until 1937, when he entered the attorney general's office.
Fay joined the Department of External Affairs in 1941 with the rank of first secretary. He served in Dublin until 1946, dealing with protocol and trade matters, and was secretary of the Irish high commission in London (1946–7). As part of the postwar expansion of the Irish diplomatic service Fay was posted to Brussels in 1947 as chargé d'affaires en titre. He left Belgium in 1950 for a year-long posting as minister plenipotentiary to Sweden and non-resident minister to Norway.
Following the appointment of Michael Rynne (qv) as ambassador to Spain, Fay returned to Iveagh House to take over as legal adviser to the Department of External Affairs, a post ranked at assistant secretary level. Fay's swift rotation of home and foreign postings continued in 1954 when he was appointed ambassador to France, where his demeanour is described in Conor Cruise O'Brien's Memoir. He moved to Ottawa as ambassador to Canada in 1960.
In 1964 Fay was appointed ambassador to Washington. Though this was a very senior Irish diplomatic posting, in Washington Irish affairs were of a lower priority to Presidents Johnson and Nixon than during the heady days of the Kennedy presidency, and Ireland's position had declined on Capitol Hill. But Fay began to build a solid network of connections in business and political circles in Washington. An area of particular importance during his tenure was the question of allowing the aircraft of US carriers, restricted to landing at Shannon airport, to have landing rights at Dublin airport. While on home leave he died suddenly 7 September 1969 at Saint Michael's nursing home, Dún Laoghaire, Co. Dublin, and was buried in Glasnevin cemetery.