Mayweather is the best defensive boxer, possibly ever. McGregor isn't a boxer. He'll not be able to land anything, get really frustrated and leave himself open, at which point he'll get knocked on his arse. It will be the ultimate rope a dope.
I give a stronger possibility of McGregor getting disqualified than winning.
The only chance he has really is that Mayweather is 40 and hasn't fought for a while. It's still a pretty small window though. If this was Mayweather in his prime, it wouldn't even have happened.