Creggs -- who had tries from Brian Donoghue, Mark Purcell and Brian Diffley, as well two Shane Purcell penalties to thank -- edged Connemara 28-27 in a thrilling encounter at the Green.
Castlebar, meanwhile, defeated Westport 6-3 in an all Mayo encounter at Carrowholly. The away team progressed thanks to two Stephen Rogan penalties.
Elsewhere, Buccaneers 2nd XV lead the Bank of Ireland Connacht J1A League after three rounds of fixtures.
The table is compact, however, and the gap between the leaders -- who have registered ten match points -- and fifth-placed Castlebar stands at only four points.
Creggs are placed second, with Westport and Connemara third and fourth respectively.
Sligo's 2nd XV, Tuam, and Monivea sit in sixth, seventh, and eighth in the standings.
In the Bank of Ireland J1B league, Ballinrobe and NUIG share top spot on 14 points with both sides boasting a 100% winning record after three rounds of fixtures.
Corrib, Corinthians 2nd XV, and Dunmore find themselves placed fourth, fifth, and sixth.
Ballina 2nd XV, Galwegians 2nd XV, and Loughrea complete the standings.
Ballyhaunis and Ballinasloe lead the way in the Bank of Ireland J1C league, with each collecting ten points after two rounds of fixtures.
Carrick on Shannon, Corinthians 3rd XV, and OLBC rank third, fourth and fifth, with Carrick and Corinthians both holding a game in hand.
Creggs 2nd XV and Tuam 2nd XV complete the standing in sixth and seventh place.
Meanwhile, in the Bank of Ireland J2 league, Galwegians 3rd XV are in top spot with eight points after two wins from their opening two rounds of fixtures.
Just behind them are Claremorris -- who also boast a 100% winning record -- while Westport 2nd XV, Oughterard and Loughrea 2nd XV find themselves third, fourth, and fifth respectively.
NUIG 2nd XV, Gort, and Sligo 3rd XV are placed sixth, seventh, and eighth, while Ballinrobe 2nd XV and Portumna complete the standings in ninth and tenth place.