[4] On 26 November 2016, Arnold made his competitive debut for Munster, starting alongside Jaco Taute in the 2016–17 Pro12 fixture against Benetton at Thomond Park.
[7] On 31 March 2017, Arnold started for Munster A in their 2016–17 British and Irish Cup semi-final against Ealing Trailfinders, scoring a try in the 25–9 win against the English Championship side at CIT.
[10] Arnold made his European Rugby Champions Cup debut for Munster on 9 December 2017, starting in the Pool 4 fixture against Leicester Tigers in Thomond Park and earning the Man-of-the-Match award in the provinces' 33–10 win.
Arnold moved to France to join Brive, where former Ireland and British & Irish Lions player Jeremy Davidson is head coach, on a three-year contract from the 2022–23 season.
[18] On 19 July 2024, Arnold returns to the UK as he signs for English side Newcastle Falcons in the Premiership Rugby on a two-year deal from the 2024-25 season.