ST ANDREWS, 19 Jul 2015 (AFP) - The 22-year-old from Greystones, a coastal town south of Dublin, carded a 66 to stun the rest of the field and shock thousands of fans assembled at the Old Course, who wondered who he was.
The last amateur to win the Open title was the legendary Bobby Jones in 1930 when amateur players were common. These days the target is usually just to make the cut.
Now the slightly-built Dunne has the chance of pulling off one of the most extraordinary wins of all time.
He ended the day tied for the lead at 12 under with the last man to win the Open at St Andrews, Louis Oosthuizen of South Africa (67), and Australian shot-maker Jason Day (67).
"It's surreal I'm leading The Open, but I can easily believe that I shot the three scores that I shot," he said.
"If we were playing an amateur event here, I wouldn't be too surprised by the scores I shot.
"It's just lucky that it happens to be in the biggest event in the world. Hopefully I can do it again tomorrow, but whether I do or not, you know, I'll survive either way."
Jordan Spieth, meanwhile, fuelled his hopes of becoming just the second player to win the first three majors of the year when he fired a third round six-under 66.
Only Ben Hogan before him has achieved the feat, in 1953, when he was 40. Spieth, who won the Masters in April and the US Open last month, is just 21.
The Texan ended the day alone at 11-under par, one shot off the leading trio.
A shot further back came two-time former winner Padraig Harrington of Ireland, who had the day's second best round of 65.
Nine players were grouped on nine-under, just three shots off the lead, including Sergio Garcia, Justin Rose, Retief Goosen and Adam Scott.
Spieth said that he intended to take no prisoners on Monday.
"I mean, at this point it's free rolling. I'm going to play to win, and I'm not playing for a place. I don't want to place third tomorrow. I want to win."
It was an extraordinary day of extravagant shot-making that saw a barrage of birdies assail the Old Course.
At one stage 10 players were tied for the lead at 10 under, with nine others a shot further back as they traded blows at the Home of Golf.
Dunne, briefly took up the pace 46 holes into the tournament, and there were charges from players who many thought were well past their sell-by dates like Harrington and Goosen.
And it was a truly international mix with Europeans, Americans, Australians, South Africans and at one stage even an Indian battling it out.
The scoring was phenomenal.
It was as if all the pent-up frustrations stored from the weather-induced inactivity of Saturday was being unleashed by the 80 cut-making players onto the fabled Fife links.
Johnson, the leader after the first two rounds, opened the day with a one-shot in front, but as he calmly parred his way down the front nine, he was quickly engulfed in a sea of red figures.
Spieth, the man he handed the US Open to at Chambers Bay by three-putting at the last led the charge and it seemed to unnerve the big American.
He reached the turn in one over 37 and at that stage he was three behind playing partner Danny Willett who had been one back at the start of the day.
By the end of the round he had spiralled down the leaderboard - a victim of his own passivity as much as the shot-making brilliance of his rivals for golf's greatest trophy.
"I just played my game. I'm aggressive when I can be and not when I can't," he said.
"You can't really worry about what everybody else is doing, you can only control yourself. I just tried to play my game all day, and I did. I just didn't hole any putts."
Just the second-ever Monday finish to an Open Championship is set up to be one of the most exciting of all times.
Collated third round scores and totals in the 144th Open Championship at St Andrews on Sunday:
(GBR and IRL unless stated, par 72; a = amateur)
204 - Paul Dunne (a) 69 69 66, Louis Oosthuizen (RSA) 67 70 67, Jason Day (AUS) 66 71 67
205 - Jordan Spieth (USA) 67 72 66
206 - Padraig Harrington 72 69 65
207 - Zach Johnson (USA) 66 71 70, Adam Scott (AUS) 70 67 70, Retief Goosen (RSA) 66 72 69, Danny Willett 66 69 72, Robert Streb (USA) 66 71 70, Justin Rose 71 68 68, Marc Leishman (AUS) 70 73 64, Sergio Garcia (ESP) 70 69 68, Jordan Niebrugge (a) (USA) 67 73 67
208 - Charl Schwartzel (RSA) 67 72 69, Steven Bowditch (AUS) 70 69 69, Eddie Pepperell 72 70 66
209 - Hideki Matsuyama (JPN) 72 66 71, Ryan Palmer (USA) 71 71 67, Stewart Cink (USA) 70 71 68, Anthony Wall 70 71 68, Dustin Johnson (USA) 65 69 75, Marc Warren 68 69 72, Rickie Fowler (USA) 72 71 66, Patrick Reed (USA) 72 70 67
210 - Matt Jones (AUS) 68 73 69, Jim Furyk (USA) 73 71 66, Paul Lawrie 66 70 74, Brooks Koepka (USA) 71 70 69, Greg Chalmers (AUS) 70 71 69, Ashley Chesters (a) 71 72 67, Anirban Lahiri (IND) 69 70 71
211 - Jason Dufner (USA) 73 71 67, Geoff Ogilvy (AUS) 71 68 72, Martin Kaymer (GER) 71 70 70, David Lingmerth (SWE) 69 72 70, David Duval (USA) 72 72 67, Webb Simpson (USA) 70 70 71, Hunter Mahan (USA) 72 72 67, Ben Martin (USA) 74 70 67, Andy Sullivan 72 71 68, Jimmy Walker (USA) 72 68 71, Luke Donald 68 70 73, Marcus Fraser (AUS) 74 69 68
212 - Oliver Schniederjans (a) (USA) 70 72 70, John Senden (AUS) 72 72 68, Greg Owen 68 73 71, Romain Langasque (a) (FRA) 69 72 71, Graham Delaet (CAN) 71 73 68, James Morrison 71 71 70, Harris English (USA) 71 72 69, Kevin Na (USA) 67 75 70, Phil Mickelson (USA) 70 72 70, Russell Henley (USA) 74 66 72, David Lipsky (USA) 73 69 70, Rafael Cabrera-Bello (ESP) 71 73 68
213 - Brett Rumford (AUS) 71 71 71, Richie Ramsay 72 71 70, Ernie Els (RSA) 71 73 69, Brendon Todd (USA) 71 73 69, Gary Woodland (USA) 72 70 71, Thongchai Jaidee (THA) 72 71 70, Lee Westwood 71 73 69
214 - Graeme McDowell 72 72 70, David Howell 68 73 73, Branden Grace (RSA) 69 72 73, Jamie Donaldson 72 71 71, Matt Kuchar (USA) 71 73 70, Henrik Stenson (SWE) 73 70 71
215 - Mark O'Meara (USA) 72 72 71, Billy Horschel (USA) 73 71 71, Bernd Wiesberger (AUT) 72 72 71, Cameron Tringale (USA) 71 71 73
216 - Francesco Molinari (ITA) 72 71 73, Thomas Aiken (RSA) 75 69 72, Ross Fisher 71 73 72, Paul Casey 70 71 75
217 - Bernhard Langer (GER) 74 70 73, Ryan Fox (NZL) 72 69 76, Scott Arnold (AUS) 71 73 73