His professional path began in England. Warwickshire County Cricket Club gave him a trial when he was 17, an unusual opportunity for a youngster from outside the traditional cricketing powers. After performing well for the county's second XI, he earned a first-team call, and his first-class debut against Somerset in September 2015 was memorable: his first wicket was Marcus Trescothick, and he finished unbeaten in both innings, almost carrying Warwickshire to a win. Competition for pace-bowling spots grew, with names such as Olly Stone joining the ranks, and the county let him go when the 2017 season ended.
Ireland had first noticed him in July 2016, when head coach John Bracewell, who praised the skills he had shown in limited chances, named him for a five-match ODI series at home against Afghanistan. Adair did not get a game in that series, and with Boyd Rankin and Peter Chase ahead of him, he was left out of later tours.
His international breakthrough eventually came in May 2019 with an ODI debut against England. Although he took no wickets that day, he did enough to receive T20I and Test caps within roughly two months, and Cricket Ireland handed him a central contract only a little over three months after his first appearance. He has since been a regular presence for Ireland across all three formats, and in 2021 he reached 50 T20I wickets in just 28 matches, the third-quickest to that mark.