Scannell secured a development contract with the senior Munster for the 2013–14 season in April 2013. He made his senior Munster debut on 21 December 2013, coming on as a replacement against Scarlets in the Pro14. Scannell made his first start for Munster on 27 December 2013, against Connacht. He signed a one-year development contract extension in March 2014. In March 2015, Scannell signed a two-year contract with Munster. Scannell made his European debut on 12 December 2015, starting against Leicester Tigers. Scannell signed a contract extension with Munster in January 2017. On 25 April 2017, it was announced that Scannell had been nominated by his teammates for the 2017 Munster Rugby Senior Player of the Year, alongside John Ryan and Tyler Bleyendaal. He signed a two-year contract extension with Munster in December 2018, a deal that will see him remain with the province until at least June 2021. Scannell earned his 100th cap for the province in their 39–22 defeat away to French club Racing 92 in round 5 of the 2019–20 Champions Cup on 12 January 2020, and he captained Munster for the first time in their 2019–20 Pro14 round 12 fixture away to Italian side Zebre on 21 February 2020, though Scannell left the field injured during the first-half of Munster's 28–0 win.
Ireland
Scannell won a total of eight caps for the Ireland Under-20 team, playing in the 2012 Under-20 Six Nations Championship and captaining the team at the 2012 IRB Junior World Championship. On 23 January 2017, Scannell was named in the Ireland squad for the opening two rounds of the 2017 Six Nations Championship. On 11 February 2017, Scannell made his debut for Ireland when, due to captain Rory Best's illness, he started against Italy in the Six Nations. Scannell was also selected in the squad for the 2017 Summer Tour against the United States and Japan. On 10 June 2017, in the one-off test against the United States, Scannell scored his first try for Ireland, doing so in the 55–19 win in the Red Bull Arena, New Jersey. When Scannell's brother Rory came on in the test as a replacement, they became the first Munster siblings to represent Ireland in the professional era. Having been called-up to replace captain Rory Best in Ireland's squad to tour Australia, Scannell went on to start in the second and third tests in Ireland's historic series victory. Scannell was selected in the 31-man Ireland squad for the 2019 Rugby World Cup, having featured in the warm-up matches against Italy and Wales.
