Michael won the Open race on his MD Racing BMW, with early leader Sheils runner-up, followed by McGee.
McGee ended William Dunlop's recent domination of the Supersport class by taking the 600cc honours, with the Ballymoney rider forced into second.
Sheils produced a superb ride in the Grand Final to see off the challenge of Michael, William and McGee.
The Cookstown BE Racing Suzuki rider overhauled early leader Michael Dunlop after making the correct choice of tyres.
Michael, 27, had reeled in the Dubliner in the earlier seven-lap Open race to take victory.
Magherafelt's Paul Jordan completed the podium in the Supersport race with Skerries rider Michael Sweeney fourth.
Jordan won the red-flagged Supertwins event after the race was red-flagged because of an incident involving Sweeney, who was unhurt.
Michael Dunlop will now head to the Isle of Man to compete in this week's Southern 100 races on the Billown circuit, near Castletown.
Michael Dunlop, Derek McGee and Derek Sheils all secured successes in Sunday's Walderstown road races.