Yet, they only managed to breach the Shannon try-line once throughout the game - albeit with an excellent try from Fanning which came about from a Grissing break.
The match was essentially decided in the first ten minutes of the second half. The teams were level at 10-10 but Shannon took two try-scoring opportunities in their stride with one fantastic try coming from ex-Bucaneers man David Quinlan and another from flanker John O'Connor.
At 22-16 with ten minutes to go, St. Mary's were still delivering their quick, fiery brand of rugby which almost took them over the line.
However, on the back of a knock on from McCarthy, centre Fionn McLoughlin collected possession and his pace allowed him to out run home full-back Gavin Dunne to the line to give Shannon a well-earned bonus point victory.
Saints director of rugby Peter Smyth admitted that Shannon proved a point to his side, saying: "We got a lesson out there in terms of playing rugby and the concentration and intensity it takes to win this league and Shannon have shown us that.
"I thought at 10-3 we dug ourselves a bit of a hole, we got back with a great try before half-time. And then just after half-time, Shannon do what they do best - they take every opportunity that comes their way.
"But in fairness to Shannon and I know it has been said a lot, you can't give them easy opportunities because they will take every single one of them," he added.
Shannon kept their full concentration throughout the game and for Smyth this was how they edged ahead at crucial stages of the top tier clash.
"I think they had a consistency to their play which was there throughout the 80 minutes and anytime you just drop intensity, they can pounce and they can take it."
St. Mary's, the early season pacestter, are now down to fourth in the table, with Dolphin having leapfrogged over them with a win over Clontarf on Sunday.
Speaking to IrishRugby.ie afterwards, Smyth gave an honest assessment of his side's chances of gaining a top three spot and qualifying for the league play-offs.
"A home semi-final looks out of the question now. Cork Con and Shannon look like they're going take the first two places.
"So we've got the Leinster Senior Cup final this Friday coming up against Old Belvedere and then we've got the last four league games, so this is our new season starting from now."
Shannon head coach Colm Tucker Jnr had much reason to be proud of his team after a hearty performance of solid defending and sharp attacking.
"We have to be proud of that achievement. It was just a really good performance out of us and it's been three very tough weeks physically and there was great heart out of the boys," he told IrishRugby.ie.
"I thought when we did put some plays together going forward we were dangerous and some of our open field running was very good. But I think it was another huge defensive effort and I'm just very proud of the boys."
Up against tough opponents who were battling hard throughout the match, Tucker Jnr agreed that his side really had to dig deep in defence to hold their nerve.
"I think that's a very good Mary's side and we did have to do a lot of defending.
"They held onto the ball for numerous phases and we really had to put bodies on the line, we had to communicate and we really had to tackle hard to hold out on a lot of occasions."
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