Conor McGregor spared no expense for Dee Devlin's birthday this week ... putting on an epic boat party for his fiancée, while showering her with designer gifts.
The UFC superstar shared pics of his surprise bash for his lover on Tuesday -- one day before Devlin, who's pregnant with the couple's fourth child, officially turns 36 -- and you can see, the guy went absolutely all out.
McGregor strung up a ton of balloons -- some pink, some gold -- in a room on their yacht .