Dr Wikipedia seems to think that tensor fasciae latae, gluteus medius and gluteus minimus do the internal rotation of the hip. So I’ll take his word for it.
The two man IR stretch involves lying on your back and a partner putting your leg into flexion (about 90) and then internal rotation for as much range as is comfortable. You can then try and pull the leg back (externally rotation) while the partner resists the motion. This is a PNF of sorts. I’ve also had some success recently using a partner to strength both IR and ER. This gets a little complex to explain via writing

What I really don’t understand is if these capsular issues are resolvable or not.