Dermot (Gun) O'Mahony (2 April 1881 – 22 April 1960) was an Irish politician and farmer. He was first elected to Dáil Éireann as a Cumann na nGaedheal Teachta Dála (TD) for the Wicklow constituency at the June 1927 general election. He was re-elected at the September 1927, 1932 and 1933 general elections. He was re-elected at the 1937 general election as a Fine Gael TD. He lost his seat at the 1938 general election.
His father Pierce Charles de Lacy O'Mahony was an Irish Parliamentary Party Member of Parliament (MP) for North Meath from 1886–1892, and his great-grandfather Pierce Mahony (1792–1853) was a Repeal MP for Kinsale in 1837.
This ends the Fifth Dáil.