Thomas Hennessy was an Irish Cumann na nGaedhael Party politician who served as a Teachta Dála (TD) in the years of the Irish Free State.
He was first elected to Dáil Éireann in a by-election on 11 March 1925 in the Dublin South constituency, after the resignation of the Cumann na nGaedhael 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 Markiewicz, he stood as a candidate in the by-election on 24 August. He won the election, becoming the first — and only — candidate to win more than one by-election to the Dáil.
The 5th Dáil lasted only 98 days in total, and last sat on 16 August 1927, before adjourning until October. However, it did not meet again; a second general election was held in September, and Hennessy was returned to the 6th Dáil. He was re-elected again at the 1932 election, but was defeated at the 1933 election, and retired from politics.