In October 2017, Munster announced the signing of Beirne on a two-year contract, which began at the start of the 2018–19 season. Beirne was named the "Guinness Players' Player of the Season" when the 2017–18 Pro14 awards were announced in August 2018. He made his Munster debut on 7 September 2018, coming on as a replacement for Dave O'Callaghan in the provinces 2018–19 Pro14 fixture against Glasgow Warriors. Beirne made his first start for Munster on 14 September 2018, doing so in their 49–13 win against Ospreys in Irish Independent Park.