Six weeks ago I did the side splits without stretching and heard a loud pop. My hamstring has been tight since. Is there irreversible damage?

Hamstring. If it is still hurting 6 weeks after the injury i would suggest having it looked at. If it is a hamstring injury is in the muscle, they usually do well with rest and therapy. If the hamstring tendon has pulled off the bone, they will occasionally need to be repaired but most of those also do well with nonoperative treatment.