Replacements used: Denis Fogarty for Henry (48 mins), Barry O'Mahony for Hayes (61), JJ Hanrahan for O'Dea (70), Declan Cusack for Deasy (72), John Ryan for Archer, Gerry Hurley for Williams, Brian O'Hara for O'Callaghan (all 75).
Replacements used: Lewis Rawlins for Lampard, Cae Trayhern for Peebles (both 51 mins), Marc Popham for Nash (60), Jamie Sollis for Price (63), Owen Jones for Dyer, Craig Gould for M Jones (both 67), Luke Williams for P Williams (70).
The superior fitness and ball-carrying abilities of the Munster 'A' team proved crucial in the end, with man-of-the-match Paddy Butler starring in both defence and attack.
Butler's thrusting run and offload unleashed Dineen for the clinching 67th minute score and frustration got the better of Cross Keys, who ended the game with both Danny Hodge and Nathan Trowbridge in the sin-bin.
It was a spirited effort from the Welsh club whose only points came from four Jason Tovey penalties. The home defence suffocated them into a succession of errors, and they were unable to get top try scorers Kristian Baller and Trowbridge into the game.
Tovey, the Dragons ace who solved Keys' injury crisis at out-half, won the battle of the place-kickers in the opening quarter in Cork with three surefooted penalties.
The hosts fell behind to a second minute penalty from Tovey, after Duncan Williams was blocked down and Munster 'A' were ruled offside in the next phase.
Denis Hurley helped secured possession from the restart, and Costello's charges won a subsequent penalty which Deasy converted for a 3-3 scoreline.
Good work at the breakdown from Keys led to Butler being pinged for holding onto the ball on the deck, and Tovey thumped over his second penalty.
David O'Callaghan drove Munster 'A' on in their attempts to hit back immediately, with Sean Scanlon countering smartly from the rear and Williams linking up play well.
Hurley finished off a crisp move in the right corner but his apparent try was ruled out for crossing, as Munster 'A' continued to carry more of threat in open play.
Deasy briefly levelled for Munster 'A', only for Tovey to make it 9-6 and reward his forwards after a surge into the home 22.
Keys captain Rob Nash and his colleagues were not making it easy for Munster 'A' at the breakdown, and a Deasy penalty miss saw the Welshmen stay in front.
But lock Hodge infringed at a ruck on the 25-minute mark, allowing Deasy to boot Munster 'A' level from straight in front of the posts.
The visitors suffered another setback when flanker Nash saw yellow for trying to interfere with Williams as he took the ball away from a ruck.
Munster 'A' wasted little time in making their numerical advantage count, with Billy Holland's close-in lineout sending Williams sniping past Hodge and Rhys Peebles and away to score a fine 27th minute try.
Deasy converted to put some breathing space between the sides, and strong carries from Brian Hayes, skipper Holland and Dave Kilcoyne kept the men in red on the front foot.
However, Keys dug their heels in and the numbers were evened up when full-back Scanlon, standing in an offside position, intercepted a pass as Keys took a quick tap.
Tovey, having missed his previous attempt from the kicking tee, fired over the resulting penalty to close the gap to 16-12 for the break.
Tellingly, the margin was pushed out to 11 points just four minutes after the resumption. The abrasive Butler led the Munster 'A' charge as they took a firmer control of proceedings and wore down the Keys defence.
The ball was turned over in the visitors' 22 but O'Dea managed to rip the ball free from Jevon Groves' grasp to dart over in the corner for an opportunist try and Deasy added the extras.
Knock ons and turnovers blighted Keys' play and they endured a scoreless third quarter with Tovey missing the target with a poorly-hit penalty.
The impressive Gareth David threatened from deep but Keys let their discipline slip, allowing Deasy to drill another penalty over for a 26-12 advantage.
Munster 'A' were forced to defend after Rhys Dyer's clever grubber kick caused problems near the try-line. Yet, Butler picked off an excellent turnover for Deasy to clear the danger.
Both benches were emptied as the sides continued to battle hard for possession with the breakdown area still fiercely competitive. Keys were hanging on but Munster 'A' made certain of the win with a terrific third try.
Replacements Barry O'Mahony and Denis Fogarty spearheaded a lively counter attack, breaking out of the home 22.
Holland was on hand to provide Deasy with quick ruck ball and he found Butler who stepped off his left and charged forward to pass for the onrushing Dineen to sprint in to left of the posts.
Aided by a rock solid set piece game, Munster 'A' had little difficulty in seeing out the result as they became the first Irish province to claim the cross-border crown - they were runners-up to Cornish Pirates in the inaugural year of 2010.
The hosts probed for a fourth try but Cross Keys, who still have the Swalec Cup title in their sights, battled on bravely.
The Welsh outfit kept Munster 'A' out despite losing Hodge to the sin-bin in the 75th minute. To add salt to their wounds, Trowbridge followed him four minutes later for a tip tackle on Danny Barnes.
Referee: James Matthew (Scotland)