Cristiano Ronaldo -- UFC Star Wore My Undies!

Cristiano Ronaldo is one step closer to UNDERWEAR DOMINATION -- 'cause a huge UFC star rocked a pair of the soccer stud's undies to a pre-fight weigh-in this week ... where things got thiiis close to turning violent.

It all went down in Ireland ... where Conor McGregor is set to take on Diego Brandao at UFC Fight Night 46 on Saturday.

McGregor came out and flashed his pair of blue CR7s ... straight from Ronaldo's male underwear line.

But moments after he stripped down, things got intense between McGregor and Brandao ... with Conor threatening to take Diego's head off during the fight.

Don't worry, the underwear emerged unscathed.