O'Mahony's Munster colleague Donnacha Ryan also excelled in the forward exchanges along with tigerish tighthead Tadhg Furlong, while Jonathan Sexton was at his talismanic best, driving Ireland to victory thanks to a stunning 46-metre penalty goal, and his half-back partner Kieran Marmion also shone on his first Six Nations start.
Sexton's memorable 62nd-minute strike proved to be the match-winning score in wet conditions. The star out-half had booted a first half penalty and converted a 23rd minute score from second row Iain Henderson, who stretched out an arm for his third try of the Championship, as Ireland established a 10-3 half-time lead.
Already confirmed as champions, England hung in there with three successful penalties from Owen Farrell, the last of them setting up a nail-biting final 13 minutes. However, Ireland made sure they ended the game in English territory with new cap Andrew Conway and fellow Six Nations debutants Luke McGrath and Dan Leavy all playing their part.
It was the third time Ireland have thwarted an English Grand Slam bid on the final day - they did so previously in 2001 and 2011 - and the defeat also ended England's 18-match unbeaten run, a tier one record they share with New Zealand and which stretched back to October 2015.
Speaking afterwards, Ireland captain Rory Best said: "We knew that England were a massive side and we knew that we were going to have to produce a really big performance. The boys fronted up, 1 to 23, and the crowd here, as always, were absolutely fantastic.
"And every time England got momentum, the crowd lifted and it was a real big lift for us to stop that. It's something we talked about, the massive bench we have to come on, and make the effort. The boys came on and we fronted up."
O'Mahony commented: "(The performance) came from a special group of guys, we were disappointed with the way some of the results have gone but we knew deep down there's a lot of heart there.
"Fellas showing up for each other and fellas showing up for what is special to them, not just on the pitch but off it as well. It was a great performance. (Losing to Wales) broke our hearts really.
"The support we have every time we come here and abroad is amazing and we let a lot of people down so thank you for today. It was a massive one for is, we needed to show everyone how much it meant to us and I think we did today."
A minute's silence was held beforehand for Captain Dara Fitzpatrick, who tragically died off the coast of Mayo earlier this week, and the brave crew of Coastguard Rescue 116. The big screen showed photos of Fitzpatrick and her missing colleagues Mark Duffy, Ciaran Smith and Paul Ormsby.
Events in Paris - where France's late win over Wales came in the 100th minute - delayed the start of proceedings in Dublin, and the returning Jared Payne, who filled Rob Kearney's shoes at full-back, showed signs of rustiness when dropping two early high balls.
The Ulster star hardly put a foot wrong after that, however, and although England had the initial territory, some well-timed tackles from Robbie Henshaw, who had another physical outing in the number 12 shirt, and the likes of Henderson and Ryan caught them behind the gain-line.
The greasy ball was proving problematic for both sides, but a well-executed maul brought Ireland up to the edge of the English 22. After Jack McGrath did well to seize a turnover on the deck, deft hands from Best and Payne almost created an eighth minute try for Keith Earls who unfortunately knocked on in contact.
Play was called back for a penalty in front of the posts and Sexton duly delivered the opening points, 10 minutes in, before a decent spell from England was rewarded with a levelling kick from Farrell after O'Mahony was penalised for not rolling away.
Skipper Best's return from a HIA coincided with Ireland getting back on the front foot, some terrific clearing out from Sean O'Brien, Henderson and Furlong allowing Marmion the quick ruck ball that he thrives on at Connacht. Garry Ringrose, who was described post-match as 'a revelation' by Joe Schmidt, was growing into the game as an elusive runner.
Successive penalties for taking out Marmion at a ruck - Maro Itoje and Billy Vunipola were the guilty parties - saw Sexton set up a five-metre lineout, and after O'Mahony's leap teed up a muscular maul, Henderson broke free to lunge for the try-line. TMO Ben Skeen confirmed the grounding and Sexton added the extras for 10-3.
The vocal home crowd were loving what they were seeing, with England clearly rattled by the aggression and accuracy of Ireland's play. Best's presence in midfield then attracted a couple of defenders, allowing Furlong and Payne to free up Ringrose for sharp burst over halfway and his pass put Earls into open space. The retreating Elliot Daly did just enough to bring the Limerick man to ground and foil the attack.
Marmion returned the favour with a vital tackle on Daly who looked dangerous as he broke onto Farrell's inviting chip over the top. A well-won ruck poach by Best saw Ireland build from their own 22, and the grunt of the back row trio, who all had eight carries by the half hour mark, continually tested England around the fringes.
Ireland dummied to set up a maul and only Marmion's slightly forward pass prevented Ringrose, who was sprung through a gap just outside the 22, from threatening the English try-line. Although Eddie Jones' charges gained turnover ball from defending a maul, O'Brien, Henshaw and Sexton combined to hold up Farrell in a choke tackle and quickly win possession back.
Having briefly threatened before the interval from a Jonathan Joseph kick that found touch in the Irish 22, England craved a strong start to the second period but it was Ireland's midfield duo of Ringrose and Henshaw who were making tough yards.
Conway, who came on from Earls at the break, showed his eagerness on kick chases, but turnover ball from an Irish maul and a needless penalty conceded by prop McGrath allowed Farrell to split the posts with 50 minutes on the clock.
England's pack, with a few reinforcements, was beginning to make headway in the maul, driving Ireland backwards and winning successive penalties, but Sexton and Henshaw's choke tackle on James Haskell broke up the visitors' momentum.
Ireland roared back on the attack as the hour mark approached, Marmion's boot nudging England backwards and then Payne countered brilliantly, slithering out of one tackle and shrugging off Ben Youngs' attempted challenge to bring the men in green right back up to the English 22.
Billy Vunipola went unpunished for a high tackle on Payne but Itoje was whistled up for offside a few minutes later, allowing Sexton to restore Ireland's seven-point advantage with the best place-kick of the evening, far out on the left wing.
A mistimed tackle by Conway on Ben Te'o, coupled with a penalty-winning maul, saw Farrell respond off the tee in the 66th minute, yet Te'o's time on the pitch was short-lived as a thunderous hit on him by Henshaw led to the former Leinster centre being replaced.
As the rain came down, a harsh call against Henshaw at a ruck set up a lineout opportunity for England on the Irish 22-metre line, but O'Mahony soared to pinch Jamie George's throw and the chance was gone for the table toppers.
It was Ireland's youthful bench that had the greater impact in the final five minutes, the fresh legs of Leavy (22), Niall Scannell (24) and John Ryan (28) helping to secure set piece ball - a penalty coughed up by Danny Care also aided them. Then came a bulldozing carry from Cian Healy before a peach of a kick from scrum half McGrath pinned England back just five metres from their own line.
England did get the ball back before the 80 minutes had elapsed but they were unable to break out of their own half, with Ireland's unrelenting defence forcing a final knock-on. It will be a great source of pride that they kept England, the back-to-back Six Nations winners, try-less in Dublin, just as they did against France last month.