Caoimhín Kelleher was born on 23 November 1998 in Cork, Ireland. He came through local clubs Rockmount and Ringmahon Rangers before joining Liverpool's academy in 2015. After progressing through the youth ranks, he made his senior competitive debut for the club in September 2019, keeping a clean sheet against Milton Keynes Dons in the EFL Cup.
Kelleher spent the majority of his Liverpool career as backup to Alisson Becker, but carved out a reputation as a reliable shot-stopper and one of the club's most effective penalty-shootout specialists. He scored the decisive penalty in Liverpool's 2022 EFL Cup final win over Chelsea and went on to set club records for the most penalty shootout saves (6) and shootout wins (4). He also started the 2024 EFL Cup final, keeping a clean sheet in a 1–0 victory over Chelsea. During his time at Anfield, he collected a Premier League title, the FA Cup, the UEFA Champions League, the UEFA Super Cup, and two EFL Cup winners' medals across 67 appearances.
A composed, penalty-saving goalkeeper with strong reflexes, Kelleher joined Brentford in June 2025 for a reported fee of £12.5 million, signing a five-year contract to take on a first-choice role in their Premier League squad.
On the international stage, Kelleher made his senior debut for the Republic of Ireland in June 2021 and has gone on to represent the nation in the UEFA Nations League, including a notable penalty save against Finland in November 2024.