Cork Constitution scored a late try to secure their ticket to the AIL Division One final where they will play Garryowen.
UL Bohemians looked on course for victory before Cronan Healy intercepted a pass to crush those hopes with a 78th-minute try at Temple Hill.
It was rough justice on the Limerick side who had led 11-6 at half-time; Martin McPhail crossing the line for a try.
Flanker Tommy O'Donnell then scored his side's second try in the 64th minute but a lapse in concentration 14 minutes later sealed the win for Cork Con.
Despite trailing 15-3 at one stage, Garryowen worked hard to get back on terms and then edge ahead of Clontarf at Castle Avenue.
Clontarf winger Derek Keane scored two first half tries as his side led 15-3 but in injury time Paul Neville touched down.
Garryowen trailed 15-8 at half-time.
A string second half showing, from the pack in particular, paved the way for Garryowen to overturn their opponents.
Hooker Damien Varley and scum-half Gerry Hurley both scored tries while out-half Eoin Hickey was solid and assured with his kicks, contributing 13 points.
Results
Division One semi-finals
Cork Constitution 21-18 Bohemians
Clontarf 15-28 Garryowen
Division Two final
Old Belvedere 13-6 Greystones
Division Three final
Wanderers 12-18 Bruff
