Beirne made nine appearances for the
Ireland under-20s 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. Beirne 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. During the World Cup itself, Beirne featured off the bench in Ireland's opening 27–3 win against
Scotland and their shock 19–12 defeat against hosts
Japan, and started in the 35–0 win against
Russia and 47–5 win against
Samoa in their final pool game, before featuring off the bench again in the comprehensive 46–14 defeat against
New Zealand in the quarter-finals, which brought an end to Ireland's 2019 World Cup. During the delayed
2020 Six Nations Championship, Beirne started in the 50–17 win against
Italy and the 35–27 defeat against
France, then featured off the bench in the
Autumn Nations Cup-opening 32–9 win against
Wales and started in the 23–10 win against
Georgia. The
2021 Six Nations saw Beirne secure his place in Ireland's starting XV, as he started in the 21–16 defeat against
Wales, the 15–13 defeat against
France, the 48–10 away win against Italy, the 27–24 away win against Scotland and the 32–18 win against
England in the final round. Beirne was named in the
2021 Six Nations Team of the Championship, having also been nominated for the
Player of the Tournament. During the
2021 Autumn Nations Series, Beirne started in Ireland's 60–5 win against
Japan on 6 November, before featuring off the bench in the famous 29–20 win against
New Zealand on 13 November. After
Iain Henderson was injured minutes before kick-off, Beirne was promoted from the bench to start against
Argentina in Ireland's final fixture of the series on 21 November, scoring a try in their 53–7 home win. Beirne started in Ireland's 29–7 win against
Wales in their opening fixture of the
2022 Six Nations Championship on 5 February, their 30–24 defeat away to
France on 12 February, their 57–6 home win against
Italy on 27 February, their 32–15 away win against
England on 12 March, and the 26–5 home win against
Scotland on 19 March that secured the
Triple Crown for Ireland. Despite having been out with a thigh injury since the Scotland fixture, Beirne was selected in the squad for the
2022 Ireland rugby union tour of New Zealand, and started in the 42–19 defeat against
New Zealand in the first test on 2 July. Beirne also started in Ireland's historic 23–12 win in the second test on 9 July, in what was Ireland's first ever away win against New Zealand, and in the 32–22 third test win against New Zealand on 16 July that secured Ireland's historic first ever series win in New Zealand. During the
2022 Autumn Nations Series, Beirne started for Ireland in their 19–16 win against world champions
South Africa on 5 November, the 35–17 win against
Fiji on 12 November, and the 13–10 win against
Australia on 19 November. Beirne was named in
World Rugby's ''Men's 15s Dream Team of the Year'' at their annual awards held in
Monaco in November 2022. Beirne continued to start for Ireland in their opening fixture of the
2023 Six Nations Championship against
Wales on 4 February, being replaced by
Ulster's Iain Henderson during the second-half of Ireland's 34–10 away win, before also starting in the 32–19 win against
France in round two on 11 February, though Beirne left the field with an ankle injury during the second-half. That injury required surgery which ruled Beirne out for 12 weeks. Beirne returned to the Ireland training squad for the
2023 Rugby World Cup warm-up matches, featuring as a replacement in the 33–17 win against
Italy on 5 August before starting in the 29–10 win against
England on 19 August and the 17–13 win against
Samoa on 26 August. He was selected in Ireland's 33-man squad for the
2023 Rugby World Cup, starting in their opening fixture of the tournament against
Romania on 9 September, scoring two tries in Ireland's 82–8 win, the 59–16 win against
Tonga on 16 September, in which Beirne scored a try, the 13–8 win against
South Africa on 23 September, the 36–14 win against
Scotland on 7 October, and the 28–24 defeat against
New Zealand in the quarter-finals on 14 October. In November 2024, he was named at lock in the
2024 World Rugby Dream Team of the Year. In November 2025, he was initially shown a controversial yellow card in the opening fixture of the
2025 Autumn Nations Series against New Zealand fly half
Beauden Barrett. During this match, this was upgraded to a 20-minute red card with Ireland going on to lose 26–13. However, after further review by the foul play officers, the red card was rescinded, with Barrett himself offering to support Beirne throughout the citing process, and he was free to play in the next game. Later that month, he was named at lock in the
2025 World Rugby Dream Team of the Year. ==British & Irish Lions==