I have a colostomy and irrigate it every day. The idea behind irrigation is to "train" the remaining colon (sigmoid or descending, not transverse) to have regular bowel movements. It is said ileostomies' output cannot be regulated and therefore they are not appropriate for regular irrigation. I can't comment on "emergency" irrigation to address a blockage like a mushroom.
I've also never been able to use a cap because there are days my colon makes its own decision to "let go" in between times even though I'm into my 5th year of irrigating. I use a Hollister mini-pouch, which is pretty much like wearing nothing at all (most of the time).
Phoenix magazine has an article this month on irrigating. It is written by a Brit and it appears parts of the technique differ between the UK and the USA. Pinky :)