Time for a bit of clarification.
Especially for all you traineded strikey peoples.
A trained strike is a pull, an all out, your going bend over and touch your toes, and I'll still be punching strike is a push. In short, if someone does a trained strike, the Maai (distance) is much shorter, and not conducive to tenkan. Particularly if you have someone trained to pull the punch in after they put it out. A snapper, so to speak.
A guy who is so drunk he can't tell if he's hitting you or santa claus is probably going to serve you up something so football tackle oriented, that pulling off an irimi on him becomes impossible.
I had a demo a while back with a guy from a dojo I am visiting once a week. He attacks like small angry bear. I wanted to do munetsuki iriminage, to show them a technique that wasn't in their curriculum, (direct entering version.)
His punch was so dominating and blind, that I found myself throwing irimi while sliding backward to maintain the proper maai. A backwards entering throw! Please don't do this at home. I hope nobody in the audience saw me.
Choosing Irimi and Tenkan is all about the distance you have been given to cross before the attack is over. Maai