William Ó Mullally or Ulliam Ó Maolalaidh was the Church of Ireland Archbishop of Tuam, 1573-95.
William Ó Mullally was a member of the Ó Maolalaidh family of Tullaghnadaly, some five miles north of Tuam. The first of the family to settle in the area was his grandfather Seán Ó Maolalaidh (fl. 1419–1480), Chief of the Name, originally from Máenmaige.
O'Mullally was a kinsman of Conchobair Ó Maolalaidh successively bishop of Clonfert (1447–1448), bishop of Emly (1448–1449) and bishop of Elphin (1449–1468).
William Ó Mullally was educated at New Inn Hall, Oxford. He was Dean of Tuam from 1558 to 1572, during which time, according to The History of the Popes,
held the rectory of Athenry, and the prebend of Laccagh, by letter of the Privy Seal dated November 6th, 1572. he was raised to the archbishopric by patent dated April 14th; and held the see of Enachdune "in commendum". Harris relates that not content with these, he endeavoured to obtain possession of Clonfert and Kilmacduagh also, but did not succeed.
William Ó Mullaly was nominated as Archbishop of Tuam on 11 Nov 1572, and appointed by Queen Elizabeth I. He was consecrated on 14 April 1573. He was also Queen Elizabeth's Chief Commissioner for the pacification of Connaught
While it is said he died in office in 1595, and this certainly was the date he was succeeded as Archbishop, his son did not succeed to his lands until 1618.