He’s already been mentioned but Young Griffo seems to be boxing’s poster boy for fighting drunk. The great John L would have had a few instances of fighting inebriated - wouldn’t he? - though I can’t say for sure specifically.
Qawi was drinking a 5th of whiskey when he fought Foreman. Not to put Frazier down, but Dwight did a lot better than Joe did twice, and the Buzzsaw was fat dull and out of shape.
Probably because it relaxed him. (Ironically, he changed his name from Braxton to the Muslim Qawi, so according to Islamic teachings, he wasn't supposed to be drinking!)
Harry Greb was reported to have had ale after weighing in for Walker, and "felt great." But he wasn't drunk. As for Mickey himself, he wrote about a fight he was going to be in that was cancelled, so he went to a bar, got plastered, but Doc Kearns managed to salvage the bout, then Walker had to go into the ring drunk as a skunk. He wrote about countless knockdowns between the two of them in that crazy fight before the vastly superior Toy Bulldog finally prevailed. (I haven't read it in a very long time, but I believe he identified the specific fight and opponent.)