Thomas Hennessy was an Irish Cumann na nGaedheal politician and medical practitioner.
He was first elected to Dáil Éireann at a by-election on 11 March 1925 for Dublin South, after the resignation of the Cumann na nGaedheal Teachta Dála (TD) Daniel McCarthy.
He did not contest the June 1927 general election, but after the death of the Fianna Fáil TD for Dublin South, Constance Markievicz, he stood as a candidate in the by-election on 24 August 1927. He won the election, becoming the first – and only – candidate to win more than one Dáil by-election. The 5th Dáil had last sat on 16 August 1927, before adjourning until October. However, on the day the result of the by-election was announced, W. T. Cosgrave sought a dissolution of the Dáil, leading to a a second general election was held in September. As of 2026, at just 63 days, the 5th Dáil is the shortest to date. Hennessy was returned to the 6th Dáil. He was re-elected again at the 1932 general election, but was defeated at the 1933 general election, and retired from politics.