Irwin was ninth in race one, won by Leon Haslam, before coming in runner-up to PBM Ducati team-mate Shane Byrne.
Alastair Seeley was 17th in the first race for Tyco BMW but Michael Laverty failed to finish in both events.
Eglinton rider David Allingham was second behind Tarran Mackenzie in Monday's feature Supersport race.
Yamaha-mounted Mackenzie maintained his 100% winning start to the season by overtaking early leader Allingham and has now collected a maximum 150 points from six outings.
Allingham lies 54 points behind the Scot in second place in the standings, with fellow Northern Ireland riders Andrew Irwin and Keith Farmer fourth and fifth in the series respectively.
Carrickfergus rider Irwin, brother of Glenn, had clinched a second position in Sunday's sprint race, with Allingham third and Farmer fourth.
Haslam continues to lead the Superbike class despite crashing in race two in an incident which also saw James Ellison and Seeley come off.
Laverty and his machine were both recovering from the after-affects of a spectacular crash in practice.
Eugene McManus was second in the Motostar event, with Richard Kerr fifth and Scott Swann 26th.
McManus is third in the championship and Kerr fifth.
Michael Dunlop continued his preparation for the North West 200 and Isle of Man TT by taking 19th in Superbike race one on his Bennetts Hawk Racing Suzuki, but failed to finish the next race.
Carl Phillips, backed by Dunlop's MD Racing Team, was a non-finisher in the Superstock 1000cc class.
Glenn Irwin finished second in race two at the third round of the British Superbike Championship at Oulton Park to move up to fifth spot in the series.