Michael and William Dunlop were in top form at the 70th Anniversary AXA Bikecare Skerries 100 meeting in north Dublin, which was hampered by heavy rain.
The feature Superbike race was called off late in the afternoon after the riders decided it was unsafe to compete due to the deteriorating conditions.
In drier conditions earlier in the day, Isle of Man TT star Michael Dunlop set a new outright lap record at Skerries of 110.871mph as he reeled in and passed home favourite Derek Sheils to win the Open Superbike race.
Dunlop came through to pip long-time leader Sheils for victory by 0.5 seconds, with older brother William taking third on the MMB Yamaha R1.
In the Supersport 600 race, William Dunlop again proved unbeatable at the National meetings on the IC Racing Yamaha R6 as he blasted into an early lead from pole.
The Ballymoney rider, who was victorious in the class at Tandragee and sealed a 600cc double at Cookstown earlier this season, had enough in hand to fend off a determined challenge from McGee by three-tenths-of-a-second as Michael Dunlop finished third on his MD Racing Yamaha ahead of Sheils.
Walderstown in Athlone hosts the next Irish road race this weekend with the 'Race of the South' feature event taking top billing. Confirmed entries include William Dunlop, Derek Sheils and Derek McGee.