JJ Hanrahan - the scorer of Munster's match-winning try in Perpignan last Saturday - turned provider for the province's latest comeback win, with his crossfield kick sending O'Mahony over in the right corner.
The young out-half added the conversion for good measure as the Scarlets' brave bid to end the RaboDirect PRO12 leaders' seven-match winning run fell agonisingly short.
Rob Evans (73 minutes) and Mike Poole (76) were sin-binned in quick succession as Simon Easterby's men scrapped for what would have been a rare win on the road - the Welsh side have only beaten Connacht away since November 2012.
Aled Thomas and man-of-the-match Hanrahan kicked a penalty apiece as the sides went in at the break on level terms (3-3), with neither try-line truly threatened.
That changed on the resumption as Munster made a lacklustre start to the second half in allowing flanker John Barclay to touch down under the posts.
Hanrahan kicked two penalties to close the gap and although he missed a 69th minute shot at the posts, the Kerry native took centre stage in another riveting rescue mission.
Despite the wintry conditions, there was a free-flowing start to the game and plenty of activity in the Munster half before Gareth Maule was wayward with a drop-goal attempt.
Hanrahan also missed the target from a seventh minute penalty after Gareth Owen prevented a quick line-out by throwing the ball away.
Ivan Dineen was prominent for the hosts in defence and attack, but the Scarlets took the lead in the 14th minute thanks to a penalty from out-half Thomas.
A well-executed line-out maul with new hooker Duncan Casey piling into the Scarlets' 22 set up Hanrahan for his levelling 23rd-minute kick.
Thomas missed a chance to move the Llanelli outfit back in front on the half hour, following good carries from Richard Kelly and busy scrum-half Rhodri Williams.
An injury to Casey brought former Ireland Under-20 captain Niall Scannell on for his competitive debut for the province, who also had to take off tighthead prop BJ Botha after he picked up a knock.
The Scarlets, who did the double over Munster last season, bolted into a 10-3 lead early in the second period thanks to Barclay's converted effort just two minutes in.
Kristian Phillips spearheaded a break from deep and Aled Thomas then found a gap between Scannell and Hanrahan, with Barclay on his shoulder to take the scoring pass.
Thomas converted and Munster were quick to respond with Hanrahan's right boot punishing Ken Owens for going off his feet at a ruck.
Although the hosts' passing let them down at times, Munster were beginning to gather some momentum and Hanrahan made it a one-point game after Josh Turnbull infringed in front of the posts.
Turnbull came to the Scarlets' rescue when he kicked the ball from Duncan Williams' grasp as Munster probed close in for an elusive try. A subsequent penalty from distance was missed by Hanrahan.
Things started to unravel for the visitors when prop Rob Evans was binned for some loose play at a Munster line-out, and then replacement Poole saw yellow for a tip tackle on Barry O'Mahony, Ronan's brother.
The breakthrough looked like it might not come as Rob McCusker marshalled an impressive show of resistance from the Scarlets, and Munster abandoned their hopes of a drop goal when Duncan Williams dummied and scampered through a gap.
The home side picked from a couple of close range rucks before Hanrahan's well-delivered kick out to the right sat up nicely for O'Mahony to score.
Television match official Dermot Moloney cleared a potential knock-on in the build-up and referee Nigel Owens was happy that O'Mahony was onside for the ruck, allowing Munster to celebrate another nail-biting victory.
There was huge relief at the rain-lashed Sportsground as Dan Parks' late drop-goal guided Connacht to a 14-11 win over Newport Gwent Dragons in the RaboDirect PRO12.
Pat Lam's men had endured eight successive defeats since beating Zebre in round one back in September, falling to the bottom rung of the league table.
But despite going 8-0 down to the Dragons inside 16 minutes, Connacht - helped by a dominant scrum and line-out - fought their way to a gutsy three-point win.
Daniel Evans' early unconverted try was the difference between the sides at the break, the wind-backed Dragons establishing a 8-3 interval lead with Jason Tovey and Parks swapping penalties.
Two more successful kicks from Parks and Tovey followed and Lyn Jones' side maintained their 11-8 lead despite losing prop Owen Evans to the sin-bin.
But Connacht produced the stronger finish with Parks setting up replacement Fionn Carr for a 75th-minute try and then nailing his vital drop goal with just over a minute left.
The sixth-minute withdrawal of centre David McSharry, who picked up an ankle injury, forced Connacht to shuffle their back-line with Carr introduced and both Robbie Henshaw and Gavin Duffy making positional switches.
It was Henshaw's missed tackle that allowed Ross Wardle to break through midfield in the eighth minute and tee up full-back Evans for his five-pointer.
Tovey, who had missed an earlier penalty, was back on target with a 16th minute penalty as the Welsh region kept pressing with a strong wind at their backs.
However, Connacht then used their solid scrum to build some momentum, with youngsters Eoin McKeon and Michael Kearney both prominent in the set-pieces.
A scrum penalty allowed Parks to knock over a 33rd minute penalty and as the strength of the wind dropped noticeably, Connacht's forwards exerted more influence with the visitors' chances to attack curtailed.
The gap was down to just two points after Parks kicked a penalty early in the second half, punishing young lock Cory Hill for being caught offside.
A good spell of possession saw the Dragons hit back with Tovey left-footing a long range penalty, but a yellow card was looming for the visitors as their own penalty count increased.
Loosehead prop Owen Evans was the man to see yellow on the hour mark, referee Alain Rolland sin-binning him for a foot trip. However, the home side suffered further frustration when Parks failed to convert the resulting penalty.
The Dragons kept it simple during a weather-beaten final quarter, with number eight Toby Faletau leading by example in terms of his sharpness on the ball.
But Connacht - driven on by man-of-the-match John Muldoon - pounced for a levelling try with five minutes left, Parks' deft crossfield kick being dotted down by the unmarked Carr out wide.
The former Scotland international narrowly missed his conversion attempt to the left, however, and Connacht needed to work their way downfield to set up a second shot at the posts for him - and he delivered.
- Sports & Recreation
- Aled Thomas
- Rob Evans
- John Barclay