Grattan married Henrietta Fitzgerald in 1782, the daughter of Nicholas Fitzgerald of County Mayo (d. 1761), a son of Thomas FitzGerald of Turlough, County Mayo and Elizabeth Browne. Henrietta's mother Margaret was the daughter of James Stephenson and Ann Price. Grattan's in-laws' history gave him a sensitivity to former Irish political difficulties, as Henrietta's Cavalier great-grandfather John Fitzgerald was transplanted from Gorteens Castle in County Kilkenny to Mayo in 1653 under the Cromwellian Settlement, and his son Thomas felt it necessary to conform to the Church of Ireland in 1717.