Complex sentence: Unfortunately this left him unable to fully devote himself to multiple tasks, and his involvement of the Irish Volunteers in 1913, and his active participation in the Irish Republican Brotherhood shortly thereafter, left St. Enda's with a less devoted master than it had had previously.
Split into simpler sentences: Unfortunately this left him unable to fully devote himself to multiple tasks. His involvement in the Irish Volunteers in 1913, and his active participation in the Irish Republican Brotherhood shortly thereafter, left St. Enda's with a less devoted master than it had previously.