The inside forwards shared 1-10 between them, with O'Regan selected as the TG4 Man of the Match for his haul of 1-6.
The Watergrasshill native set the tone early on for Denis Ring's charges in the Munster quarter-final at Páirc Uí Rinn, grabbing two high balls and turning to split the posts.
He continued to do wreck in the second half and buried a goal to propel Cork 1-17 to 0-15 ahead entering the fourth quarter after a scorching run by Evan Sheehan resulted in a shot saved by Jason Gillane.
"We're thrilled to get through the game," said O'Regan afterwards. "Limerick were a physical team and that didn't give us anything easy."
The deadly number 14 revealed it had taken a lot of hard graft to get to this level.
"I wouldn't have been involved with Cork at U14 and U15 but I just kept working away. I was delighted the management gave me a chance and I didn't want to let them, or my family or club, down."
He certainly didn't. His aerial prowess was incredible given he's under six feet tall and the ball-winning was matched by his finishing. Three of O'Regan's six points were as good as you'd see at any level.
Turnbull was back at his best, having lost the bulk of last year through a cruciate injury, twisting and turning before clipping his four scores from a variety of angles.
Sheehan might not have scored from play but was fouled for two of the three converted frees and threatened throughout.
Limerick were underdogs against a Rebel unit drawn from the core of the minor group that reached the All-Ireland two years ago but they were no pushovers. Paul O'Brien was exceptional from placed balls while Mikey O'Brien, Ronan Connolly and Mark McCarthy were always dangerous.
It was 0-10 to 0-8 to Cork at half-time, though the home side had shot six wides to Limerick's one.
Corner-back Conor O'Callaghan from Dromtarriffe was a standout in 2017 at minor and for the U17 team that captured an All-Ireland and excelled again here. James Keating was teak-tough at the edge of the square while Seán O'Leary Hayes hurled the world of ball at six.
Daire Connery finished with 0-3, including a spectacular late sideline, while sub Liam O'Shea chipped in with two points. Craig Hanifin also hit a couple of neat scores from play and Seán Twomey added a bit of heft off the bench.
Cork senior Robert Downey was at wing-back while senior panelist Ger Millerick was fit enough to come on in the second half, replacing Kanturk's Ryan Walsh who had also done well. Millerick's passing from defence was as excellent as it has been for Fr O'Neill's and Imokilly in recent years.
Elsewhere Tommy O'Connell and the Roche twins, Eoin and Brian, worked tirelessly, and Cork needed that because without the right attitude this was a game they easily could have lost.
The U20s are back in action next Wednesday night at Páirc Uí Rinn again for the provincial semi-final against Clare, with Tipp and Waterford on the other side of the draw.
Downey will first be involved with the Cork seniors in their qualifier in Westmeath this Sunday.