Thomas Barr represented Ireland in the 400 metres hurdles at the Rio 2016 and Tokyo 2020 Olympic Games, and in the mixed 4×400 metres relay at Paris 2024.

At Rio 2016 he reached the final of the 400 metres hurdles and finished fourth in a time of 47.97 seconds, setting an Irish national record that still stands. His time 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 his second fastest ever time of 48.26 but narrowly missing the final. At Paris 2024 he competed in the mixed 4×400 metres relay, with Ireland failing to advance from the heats.

Barr grew up in Dunmore East and began athletics with Ferrybank AC. He was not a standout juvenile athlete and did not break 50 seconds in the hurdles until he joined 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. Away from athletics, he is the brother of Irish Olympic track athlete Jessie Barr. He won 12 Irish national titles in the 400 metres hurdles across his career, and won bronze at the 2018 European Athletics Championships — the first Irish male sprinter to win a medal at an outdoor European Championships. He announced his retirement in January 2025.