Masterson previously played for Irish provincial side Connacht in the Pro14, after being promoted from the academy ahead of the 2015–16 season.

[3] As a teenager, Masterson was part of Irish midlands representative teams and involved in the youth set up of his native province, Leinster, where he captained the under-18 side.

[5] Having representing Scotland at the 2013 Junior World Cup, Masterson approached Connacht, his mother's native province, for a trial.

[6] After playing for the province's second tier side, the Connacht Eagles, Masterson made his debut for the senior team on 23 November 2013.

[7] He didn't feature again until 23 February 2014, when he started against Zebre in a 19–27 win, playing as an openside flanker following to injuries to senior players Jake Heenan and Willie Faloon.

[10] Ahead of the 2014–15 season, Masterson was made captain of the Eagles and started all six of Connacht's matches in the 2014–15 British and Irish Cup.

[12][13] He made nine appearances, seven of them starts, and scored his first try for the first team against Newport Gwent Dragons on 22 February 2015, adding two more during the campaign.