Blackrock College consigned Shannon to their fifth defeat in a row as the hosts triumphed 21-19 in another match that went right down to the wire.
Both of these sides are yet to confirm their place in Division 1B for next season, so it was no surprise that the first half was a tight and edgy affair.
'Rock led 9-8 at half-time thanks to three penalties from goal-kicking scrum half Jack O'Carroll, who struck the ball really well in the heavy conditions.
Shannon's left winger Greg O'Shea tore clear to score an excellent individual try, and he completed his brace out wide early in the second half.
However, despite a combination nine-point tally from penalties converted by Ronan McKenna and replacement Tadhg Bennett (2), missed kicks came back to haunt Shannon in the end.
McKenna and out-half Shane Airey both misfired on a couple of occasions and that allowed Blackrock to build pressure once again. Their well-organised maul forced a penalty try which was converted for 16-13.
Bennett squared things up for Marcus Horan's men and added another penalty in the closing stages, but in between, Blackrock flanker Sam Cooke burst free on the left wing to run in the game's decisive try.
Just two points in arrears, Shannon missed another shot at the posts in the dying minutes and 'Rock held on for a gritty victory. A similar result will be needed away to Dublin University on April 16, asEmmet Farrell's charges actually slipped to ninth place following UL Bohs' bonus point success against Belfast Harlequins.