The County Louth star (18) took time out from the Leaving Cert studies and produced an inspired performance with the blade as he made seven birdies before dropping his only shot of the day at the tough 17th.
His brilliant round gave him a one-stroke lead over two Walker Cup players in Galway’s Liam Nolan and Malone’s Matt McClean, who played together and set the early pace with matching 66s.
Berehaven’s Joe Neill sits alone in fourth after a fine 67 with 2022 winner Colm Campbell from Warrenpoint posting a three-under 68 to share fifth with Co Sligo duo TJ Ford and Ruairi O’Connor.
“I’m pretty happy with it,” said Tiernan, who followed a birdie at the third with four in a row from the fifth to turn in a blistering 31 before making two more birdies at the 10th and 15th.
“Just stuck to my strategy the whole day, was making sure I was putting myself on the right side of the pins because if you get above it it can get very tricky.
“Off the tee, I was pretty good and got a nice run of birdies. It was a good start to the week. I feel like I have the game; I am just giving it time.
“I haven’t been playing too much recently so trying to stay patient with it. Nice to shoot a good score today.”
Tiernan is getting ready to sit the Leaving Cert, but he isn’t neglecting his studies in Rosses Point.
“Leaving Cert now in a few weeks, so I brought the books with me and it’s back to study now,” said the Baltray talent, who is getting ready to head to East Tennessee State University on a golf scholarship in August.
Scottish raiders Conor Graham and Gregor Tait were in a six-man group on two-under alongside Carton House’s West of Ireland champion Keith Egan, Grange’s Jake Whelan, Royal Dublin’s Jack McDonnell and the Czech David Tomi.
However, the presence of Nolan and McClean near the top of the leaderboard is an ominous one.
“I’m absolutely delighted,” said Nolan. “I’ve been away for a little while. I knew I was playing some good golf, but it’s hard to know how it’s going to translate from the range and practising on the course to a massive championship, so I knew fairly early on that my swing was good, and I was able to roll in a few putts.”
Roganstown star Sean Keeling (17) turned in two over after a double bogey at the short ninth but rallied with three birdies to start the back nine and posted a 70 that left him tied 14th on a day when just 22 players in the 138-strong field took advantage of ideal conditions to break par.
The leading 50 and ties after three rounds will qualify for Sunday’s final round.
Flogas Irish Men’s Amateur Open Championship, Co Sligo GC (Par 71)
Round 1
65 Gavin Tiernan (Co Louth);
66 Liam Nolan (Galway), Matthew McClean (Malone);
67 Joe O’Neill (Berehaven);
68 Colm Campbell (Warrenpoint), TJ Ford (Co Sligo), Ruairi O’Connor (Co Sligo);
69 Gregor Graham (Sco), David Tomi (Cze), Keith Egan (Carton House), Gregor Tait (Sco), Jake Whelan (Grange), Jack McDonnell (Royal Dublin);
70 Graham Donohoe (Enniscorthy), Hugh Foley (Royal Dublin), Adam Smith (Mullingar), Luke Kelly (Dunfanaghy), David Howard (Fota Island), Sean Keeling (Roganstown), Max Weaver (Wal), Sam Mukherjee (Sco), David Kitt (Athenry);
71 Fionn Dobbin (Malone), Morgan Cain (Cork), Tom Matthews (Wal), Jamie Butler (Naas), Ryan Griffin (Ballybofey), Benjamin Oberholzer (Clandeboye), Gary Collins (Rosslare), Mark Doogue (Castleknock), Frederik Dortmans (Eng), Alyx Murray (Sco), Aaron Marshall (Lisburn);
72 Harry Gillivan (Portmarnock), Julius Sommer (SUI), Edward Reed (Eng), Paul O’Hanlon (Carton House), Colm Hughes (Castle), Jack Hearn (Tramore), Shane McDermott (Slieve Russell);
73 Jake Foley (Elm Park), Caolan Rafferty (Dundalk), Paul Coughlan (Castleknock), Dominic McGlinchey (Sco), James Hewitt (Tandragee), Andrew Benson (Sco), James Stenson (The Island), Sean Reddy (Douglas), Michael Roche (Eng), David Brady (Straffan), Cian O’Connor (Roscommon), David Reddan (Nenagh), Darryl Gwilliam (Eng), Thomas Abom (Edmondstown), Shea Dorman (Bangor), Seán McLoughlin (Co Sligo), Paraic Connolly (The Links Portmarnock), Sean Cooper (Enniscorthy), Scott Spark (Sco), Oscar Murphy (Royal Portrush), Charlie Smyth (PGA National Slieve Russell), Matthew Wilson (Sco), Quentin Carew (Castleknock), Adam Allahbachani (Powerscourt), Richard Knightly (Royal Dublin), Scott Mann (Sco), Jacob Kelso (Eng);
74 Brodie Cunningham (Sco), James Temple (Portmarnock), James Fox (Portmarnock), Simon Walker (Roscommon), Ross McCormack (Palmerstown House Estate), Robbie Cannon (Balbriggan), Sean Flanagan (Portmarnock), Darragh Flynn (Carton House), Ross Laird (Sco), Ryan Williams (Wal), Aodhagan Brady (Co Sligo), Ben Best (Rathmore), Dwayne Mallon (Dungannon), Sean Murphy (Grange), Rory Gallagher (Galway Bay), Daniel Bennett (RSA), Oliver Mukherjee (Sco), Theo Baker (Wal);
75 Simon Austin (AUT), Eoin Murphy (Dundalk), Jake Adler (Nenagh), Adam Buchanan (Royal Portrush), Allan Hill (Roscommon), Christian Johnson (Eng), Jonathan Keane (Lahinch), Lorcan O’Connor (Edmondstown), Ross Nelson (Belvoir Park), Miles Gray (Eng), Andrew Mulholland (Castlerock), Marc McKinstry (Cairndhu), Cian Harkin (Letterkenny), Adam Scott (Scot), Seamus Cullen (Slieve Russell), Peter O’Keeffe (Douglas);
76 Liam Abom (Edmondstown), Robert Galligan (Elm Park), Aidan Dooley (Carton House), Ryan McNelis (Fintona), Conor Hickey (Dooks), Gordon Sillett (Ballykisteen);
77 Lewis Gowdy (Bangor), Mark Gazi (Tralee), Jordan Boles (Charleville), Caolan Burford (Wal), Alexander Farmer (Sco), Samuel Gammell (Naas), Ross Latimer (Clandeboye), Dylan Kayne (RSA),l
78 Lucas McKeen (Co Louth), Paul Carey (Nenagh), Marc Norton (Belvoir Park), Edward Rowe (Belvoir Park), David Marshall (Naas), Rory McConnell (Belvoir Park), Mark Cadden (Roganstown), John Doyle (Fota Island);
79 Robert Walsh (Douglas), Donnacha Cleary (Tipperary), Kevin Robinson (Castlewarden), Evan Farrell (Co Louth), Eoin Sullivan (Portmarnock), Colin Smith (Co Sligo), Finn Coull (Sco);
80 Conor Stapleton (Connemara);
81 Sean David (Wal), Alex Jaap (Wal), Dean O’Riordan (Fermoy);
82 John Tandberg (SUI), Aaron Foley (Edmondstown);
83 Robbie Hynes (Royal Dublin);
90 Luke Harris (Hermitage).