*Club domestic league appearances and goals, correct as of 18:58, 16 October 2024 (UTC) ‡ National team caps and goals, correct as of 00:57, 15 November 2024
From Dublin, Grehan came through the youth system at Bohemians and featured in the first team for the first time in the FAI Cup against College Corinthians in July 2021. He signed for Premier League club Crystal Palace in August 2022 following a trial.[2][3] Grehan signed a new three-year contract with Palace in December 2023.[4] In January 2024, he signed on loan with Carlisle United.[5] On 3 February 2024, Grehan made his professional debut as a late substitute in Carlisle's 3–2 defeat away to Leyton Orient.[6]
He has been described as a versatile
defender that can play on the right or left side as a centre-back, and can also pass the ball out from the back.[9]