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. On 21 April 2017, Arnold started at 13 for Munster A in their 29–28 victory over English RFU Championship side Jersey Reds in the final of the 2016–17 British and Irish Cup, which was held in Irish Independent Park. He scored his first tries for Munster on 3 November 2017, doing so in the 2017–18 Pro14 win against Welsh side Dragons. 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.