Donnell began playing mini rugby for Winchester RFC and has represented England at under-18s and also under-20s level.
Having joined the Irish Academy at 16, he went on to captain the Club's under-18s side and was chosen for his first appearance aged just 18 during the 2017/18 season, coming off the bench against Gloucester Rugby in the Anglo-Welsh Cup.
Donnell has since become a regular squad member at a tender age and made 16 appearances for London Irish during the 2020/21 season.
After featuring in the opening five rounds of the 2021/22 Gallagher Premiership season, Donnell was subjected to a spell on the sidelines due to injury.
Nonetheless, he made a triumphant return in the latter portion of London Irish's historic run to the Premiership Rugby Cup final, dotting down in the home semi-final against Leicester Tigers.
The athletic back-row has read economics at Surrey University and is on course to surpass 50 appearances for the Exiles in the 2022/23 season.