Born in Dublin, Clarkson attended Blackrock College and won a Leinster Schools Rugby Senior Cup with the school in 2018, alongside fellow Leinster academy member Liam Turner.
[2] He made his senior debut for the province on 29 August 2020 as a 63rd minute replacement for Michael Bent in their 28–10 derby victory against Ulster.
[1] He also represented the team at the 2019 World Rugby Under 20 Championship, and was vice-captain of the side during the 2020 Six Nations Under 20s Championship, where they secured consecutive triple crown's,[5] though the tournament was cancelled with two rounds still to be played due to the COVID-19 pandemic.
[6] In November 2024, he made his debut for Ireland in the 2024 Autumn Nations Series coming off the bench to replace Finlay Bealham in 22-19 win over Argentina.
[7] In January 2025, he was named on the bench to make his tournament debut for the opening round of the 2025 Six Nations.