Justin Gaethje

Conor McGregor's a living legend to his Irish fans ... but Ireland's 2nd biggest UFC star says he is NOT cut out for a political career back home. We asked "Irish" Joe Duffy about McGregor pulling a Trump when his fighting days are done -- something Conor teased when we got him out in NYC last…

Undefeated UFC star Justin Gaethje is saying sayonara to MMA after another dozen fights, tops ... telling TMZ Sports the sport's "too brutal" to gun for 50-0 like Floyd Mayweather. "I'll get to 30-0 and I'm outta here," Justin told us ... before saying there's NO WAY a UFC fighter could hit 50…