Mullarney, 23, never trailed and though he stumbled at 17, missing a six-foot putt to win the match, Mullarney emerged triumphant on 18 when Brazill failed to birdie.
"I am absolutely delighted," Mullarney gushed. "It did absolutely grate on me that I haven't won one of these before. I am delighted."
For Mullarney, this was his first experience playing in the final match of a major amateur championship. He played brilliant golf on a postcard day at Ballybunion, but made a critical error at No. 17. He was pin high there, but three-putted for a bogey. Brazill was in the rough, but he scrambled for a par to cut Mullarney’s lead from 2 up to 1 up and give himself a shot at the closing hole.
Brazill’s approach stopped 15 feet short, giving him far from an outside chance. Mullarney secured his par with ease but watched tensely what Brazill could do.
"I was certain that putt was in on 18," Brazill said ruefully. "It would have been nice to make the birdie."
When that final try failed, Brazill flung his putter into mid-air.
"I knew Ronan was never going to make a mistake," Brazill said. "He just doesn't hit it off line. He knows where it's going every single time. I know I am capable of a two-way miss, even with an iron. I don't think he knows what a miss is and he's a really good putter. He's such a hard man to play against. You really want to be on your game if you're playing him."
Information from the Golf Union of Ireland used in this report