Pierce Brosnan Says Conor's Gonna KO Floyd ... Irish Pride, Baby!!

Pierce Brosnan is ALL ABOUT the Fighting Irish on St. Paddy's Day ... telling TMZ Sports his fellow countryman, Conor McGregor, will DESTROY Floyd Mayweather if they actually square up.

FYI, Brosnan, who was leaving Nellos in NYC, was born in Drogheda -- one of the oldest towns in Ireland and about a 45 minute drive from Conor's hometown of Crumlin.

Pierce says he hasn't been back to Ireland for SPD in years -- but he still clearly bleeds GREEN when it comes to the fight game.