In the end Munster had to work hard as the home side pressed for the try that might have seen them snatch a dramatic victory. But it was defence that Costello identified as the biggest contributing factor in Munster's win.
"The most pleasing thing was our intent to play rugby." he said. "I thought we were very very positive in what we tried to do. From start to finish. Even in the first half when we didn't have a huge amount of ball. The ball we had we used really well.â0"
"At half time we acknowledged that, were really happy where our attack was at but our defence had to be overhauled. We felt we weren't physical enough, our attitude was very passive so we became a lot more pro-active on our defence, a lot more physical and for 35 minutes in the second half we nailed it (defence) and that's what won the game for us."
Munster dominated most of the second half but midway through the opening one Costello found his side floundering somewhat even though they'd the opening try to their credit.
It wasn't by any means an insurmountable lead but by then Leinster held the whip-hand and when Noel Reid kicked them into a 9-5, 33rd minute lead - it should have been more as Leinster pressed left and right - the indications were that Leinster could take control.
And then they, ever so slightly, relaxed, and JJ Hanrahan picked a pocket or two to send his side in at the break leading eleven points to nine.
Leinster deserved better for their first half efforts. Rhys Ruddock was at the core of all that was good about them but the only comparable - particularly after Tommy O'Donnell departed injured - was Paddy Butler.
As good as Mike Ruddock's son was - and he was - Butler was better. He contributed handsomely to every facet of the play as Costello readily acknowledged, "â00Paddy was outstanding. For ten minutes before he came off (back spasm) he must have made double figures in terms of tackles. He's an outstanding rugby player. For me he's really excelled since this fixture last year. He was excellent in that semi final and since then has really kicked on."
Costello also made reference to three areas that were key to the win. "In that last scrum, right in front of the posts as they looked to get a platform, Kevin Griffin, a club player outside the system under the tutelage of Paul McCarthy, came in and played out of position at loose head and locked out that last scrum."
"Duncan Williams then got a turnover near the line. I'd say he got three turnovers in that half. He made some really good decisions around defence. And then there's Felix. When guys are making half breaches, the speed Felix comes into the line with and his decision making in just coming into the line, his attitude, it's just so infectious."
"Those three things set the tone for those last six, seven minutes so that they couldn't breach, couldn't score."