Although born in Pittsburgh, Sean returned to Dublin before the age of 1, and played his junior rugby at Greystones Rugby Club in Wicklow before winning a JCT and SCT medal with Blackrock College.
Sean was part of the Leinster academy from 2020 to 2023 when he was then offered a professional contract with Connacht Rugby.[2]
O'Brien was named in the Connacht Squad for the 2023–24 season.[3] He made his debut, and received his first cap vs Ospreys.[6] O'Brien then travelled with the squad to South Africa, playing in both away games, vs the Sharks and Bulls, scoring his first URC try in Durban.[7]