Replacements: Mark O'Brien, Rhys Buckler, Michael Carroll, Jack O'Carroll, Jack Dempsey.
Replacements: Conor Glynn, Barry McNamara, James Murphy, Lee Nicholas, Dave Evans.
Two penalties from David Godfrey gave 'Rock a 13-5 advantage, but Shannon took full advantage of Peter McCague's sin-binning to score through their Munster flanker Paddy Butler in first half injury-time.
After the sin-binning of Shannon hooker Niall Horan, Blackrock increased their lead from a second Byrne score.
But Rob Guerin shrugged off two weak tackles to score under the posts and with Leader's conversion, hand Shannon the victory by the narrowest of margins.
Since their opening day clash back in September, where Blackrock prevailed 23-16 at Thomond Park, the fortunes of these two clubs have taken different turns.
While Blackrock suffered nine defeats from their next 10 league games, Shannon have recovered from a slow start to win five out of their last seven matches.
Victory therefore was vital for both teams as Blackrock were aiming to drag themselves clear of relegation and Shannon were chasing a fourth consecutive win.
The game started off in see-saw fashion and the hosts took the lead after seven minutes when centre Byrne pounced on a loose ball in midfield to run through unopposed.
Shannon, however, hit back and following a sustained period of pressure, moved the ball out wide for out-half Leader to touch down in the corner.
Two successive penalties from Godfrey stretched 'Rock's lead, but just as the first half was drawing to a close the Limerick men fought back.
Rock forward McCague was sin-binned for infringing and Shannon scored from the ensuing attack through the out-stretched arm of Butler..
The visitors put 'Rock under pressure inside their own half on the resumption, but they missed the chance to go in front when Leader's penalty attempt sailed wide of the posts.
Horan was next to go in the bin as the tide turned in the Dubliners' favour again. Byrne went over on the left in the 66th minute following a swift back-line move off a lineout which stretched the visiting defence.
But Shannon had the last laugh when Guerin broke free of two tame challenges on 71 minutes to scramble in under the posts. The conversion from Leader put Shannon ahead and they held on through a frantic finale.
Referee: Mark Patton (IRFU)