Beirne graduated through the Leinster Academy and made his senior debut from the bench against the Cardiff Blues on 12 September 2015. Beirne went on to make a further 3 appearances for Leinster, playing twice against Dragons and once against Scarlets, a team he would later go on to play for. All of these appearances came off the bench. Beirne spent most of his career at Leinster playing for their A team and he earned 10 caps for them, making 9 appearances in the British and Irish Cup. Beirne was then released by Leinster.
Scarlets
It was announced on 28 June 2016 that Beirne had moved to Scarlets. Beirne made his Scarlets debut off the bench in the opening game of the season, a loss against Munster. He then played a massive role in Scarlets successful 2016–17 season, in which they beat Munster 46–22 in the 2017 Pro12 Grand Final to win their first silverware since the 2003–04 season.
Munster
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. He scored his first try for Munster in the provinces 30–22 defeat at the hands of Leinster on 6 October 2018. Beirne made his Champions Cup debut for Munster on 13 October 2018, earning the Man-of-the-Match in the provinces opening 2018–19 pool 2 fixture against English side Exeter Chiefs, which ended in a 10–10 draw in Sandy Park. He was Man-of-the-Match in Munster's 26–17 win against Leinster on 29 December 2018 and in their 9–7 win against Exeter Chiefs in round 6 of the Champions Cup on 19 January 2019, a result the ensured Munster advanced to a record 18th Champions Cup quarter-final. Beirne signed a two-year contract extension with Munster in May 2019, which will see him remain with the province until at least June 2022. He was named in the 2018–19 Pro14 Dream Team when it was announced in May 2019, his second such accolade, having also been named in the 2017–18 Pro14 Dream Team whilst a Scarlets player. Beirne suffered a sustained a fractured ankle in Munster's 15–6 defeat away to English side Saracens in round 4 of the 2019–20 Champions Cup on 14 December 2019, and subsequently had to undergo surgery for the injury in January 2020, ruling him out for the foreseeable future.
Beirne made nine appearances for the Ireland U20 team and was part of the team which finished 6th in the 2012 IRB Junior World Championship. Beirne also played for the Ireland sevens team in 2015. Beirne received his first senior international call-up in May 2018, being selected in the Ireland squad to tour Australia in June, and made his senior Ireland debut on 16 June 2018, coming on as a replacement for Peter O'Mahony in the second test against Australia, before also coming off the bench for Ireland during their 20–16 third test victory, a win that sealed a 2–1 historic series victory for Ireland. Beirne made his first start for Ireland against Italy on 3 November 2018, doing so in their Autumn international in Soldier Field, Chicago, and scoring two tries in his sides 54–7 victory, whilst also starting in the 57–14 win against the United States three weeks later. He was selected in the 31-man Ireland squad for the 2019 Rugby World Cup, having featured in the warm-up matches against Italy, England and Wales.
