McNulty turned on the afterburners with a perfectly-timed sprint to race past Healy (EF Education-Easy Post) and Frigo (Israel–Premier Tech) in the final 100m to snatch victory.
"Indescribable. On the last long climb I tried to go. I thought my race was done there because Ben was so strong," McNulty said.
"I clawed back and rested and then we played games on the flats. In the end it came down to the last kick and the sprint.
"I knew (Frigo) was coming and he ended up coming just at the right moment because we could swing over. I caught the draft and then at 150-200 metres I just went for it."
Healy finished second, ahead of Frigo, who appeared to drop off the pace but exerted a superhuman effort in the home straight to finish inches short of a stage win.
"You never give up. I had really good legs. Of course the first two maybe had something else on me. I gave everything and they gave everything," Frigo said.
"The descent in the final gave me a chance to get back and I knew that. I played that, and that's how it went. In those moments the head is gone, is spent, but you carry on pushing."
A group of 15 riders, including former Giro d'Italia stage winner Alberto Dainese (25), built an advantage of over six minutes and 30 seconds after the category one ascent in the first half of the race.
Niccolo Bonifazio (29, Intermarche-Circus-Wanty) attacked with just under 50 kilometres left, but was quickly reeled in with 35 kilometres to go by the trio of Healy, Frigo and McNulty, who never looked back.
Monday is a rest day on the Giro. Tuesday's stage 16 is mountainous 203-kilometre ride from Sabbio Chiese to Monte Bondone, with over 5,000 metres of elevation.
