Sir Gerald or Gerard Lally or O'Mullally (? in Tuam, County Galway – 1737) was an Irish conservative and French military officer.
He was the second son of Thomas Lally (or O'Mullally) of Tullaghnadaly, by his wife, Jane, sister of Theobald Dillon, 7th Viscount Dillon, and younger brother of James Lally. He served with James II's forces in Ireland, and after the end of the Siege of Limerick (1691) he fled to France.
At Romans on 18 April 1701 he married Anne-Marie, the daughter of Charles Jacques de Bressac, seigneur de La Vache; they had a son Thomas Arthur Lally. On 7 July 1707 he was created a Baronet, in the Jacobite baronetage of Ireland, by James II's son, the titular James III.
On 28 July 1708 he was made Lieutenant Colonel of the famous Dillon's Regiment, named after his first cousin General Arthur Dillon. On 20 February 1734 he was made a Brigadier General in the French army. He died at Arras in November 1737, and was succeeded by his son.
His grandson was Gérard de Lally-Tollendal.