Born in England, he represents Ireland at international level after qualifying on ancestry grounds.
[6] He played regularly in his first season, thanks to the absence of Alan O'Connor and Peter Browne with injuries, and the departure of Dan Tuohy to Bristol Bears,[7] making 21 starts and winning 47 lineouts.
[10] In 2018–19 he established himself in the team alongside Iain Henderson, making 28 appearances including 22 starts, and 235 tackles.
[15] In the 2022 tour of New Zealand[16] which Ireland won, Treadwell played in all three tests, and in both non-test matches against the Maori All Blacks.
In the 2023 Six Nations[17] Treadwell helped Ireland win a Grand Slam by playing against his country of birth, England.