The Irish superstar sent out a random tweet Monday night raving about POTUS for no apparent reason ... maybe he was just in a super-Trumpy mood following his UFC 246 victory.
"Phenomenal President. Quite possibly the USA [Greatest of All Time]," McGregor wrote.
"Most certainly one of them anyway, as he sits atop the shoulders of many amazing giants that came before him. No easy feet. Early stages of term also. Incredible."
He added, "Congrats and Happy Martin Luther King Jr. day America."
Trump LOVED the shout-out and responded early Tuesday morning -- writing, "Congratulations on your big @UFC WIN!"
"Donald can shut his big fat mouth," McGregor said at the time ... "I don't give a f*ck about Donald Trump."
My how times have changed!