UFC's Conor McGregor -- Vegas Bikini Pool Party ... After Big Victory

UFC badass Conor McGregor kept the Irish party train rollin' on Sunday -- extending his UFC 189 victory party to a crazy Vegas bikini party ... TMZ Sports has learned.

It all went down at the Foxtail Pool Club at the SLS Hotel ... where a barely bruised McGregor took in some sun with UFC ring girl Arianny Celeste ... while Ty Dolla Sign took care of the music.

Amazing McGregor even made it out Sunday -- considering he was partying his face off the night before ... hitting up Vegas nightclubs hours after he beat up Chad Mendes in UFC 189.

Guess he can sleep on the plane.