John Irwin was born on 17 February 1740 in County Tyrone, (Northern) Ireland. In 1762 he immigrated to Pennsylvania, where he served in the British Army as a commissary. During the Revolutionary War, Irwin was the Continental Army’s deputy commissary of issues for the Western Department. He subsequently served in the Pennsylvania legislature and on the Court of Common Pleas for Westmoreland County. Irwin and wife Elizabeth Cunningham (1748–1818, m. 1766) had four children.