Thomas Barr set the Irish national record of 47.97 seconds for the 400 metres hurdles when finishing fourth in the Olympic final at Rio 2016, a mark that still stands. His run was the first by an Irish athlete in an Olympic 400 metres hurdles final since Bob Tisdall in 1932.
At Tokyo 2020 he reached the semi-finals, posting 48.26, his second fastest time. At Paris 2024 he ran in the mixed 4x400 metres relay.
Barr grew up in Dunmore East and began athletics with Ferrybank AC. He made his breakthrough after joining the training group of coaches Hayley and Drew Harrison at the University of Limerick, where he studied mechanical engineering and later completed a master's in sport science. He won bronze at the 2018 European Athletics Championships, becoming the first Irish male sprinter to win a medal at an outdoor European Championships, and took 12 Irish national titles in the 400 metres hurdles. He is the brother of Olympic track athlete Jessie Barr. He announced his retirement in January 2025.