[1] Before playing professionally, Sheehan was a Leinster Schools Senior Cup semi-finalist with Clongowes Wood College in 2016.
[10][11] In the 2022 Six Nations, Sheehan came off the bench in Ireland's win vs Wales and played the majority of the defeat to France after Rónan Kelleher came off injured in the 26th minute.
[12] In the final match of the 2022 Six Nations, he scored a try and won the player-of-the-match award in Ireland's 26–5 victory over Scotland, clinching their 12th triple crown.
[14] On 18 March 2023, Sheehan scored two tries in a player-of-the-match performance against England which clinched the 2023 Six Nations and Ireland's fourth ever Grand slam.
[15] In January 2025, despite just returning from a six month long injury and only one appearance in the URC that season, he was named on the bench for the opening round of the 2025 Six Nations against England.