Northern Ireland's US Open champion was playing in a group alongside Lee Westwood, with whom McIlroy shared a manager - Andrew 'Chubby' Chandler - until recently moving to join Ryder Cup partner Graeme McDowell at Dublin-based Horizon.
McIlroy's impressive bogey-free round gave him a one-shot lead, with American Hunter Mahan his closest challenger.
Paul Casey, Ian Poulter and Padraig Harrington shared third place, three off the lead, after rounds of 67, with American Anthony Kim and China's Li Chao carding 68s.
Westwood finished on 69, which positioned him level with John Daly, Robert Karlsson, YE Yang and Retief Goosen, while Colin Montgomerie was one further adrift, along with South Africans Charl Schwartzel and Louis Oosthuizen.
The tournament is a a 30-man invitational event, not aligned with the main tours, offering the highest first prize in professional golf.
What would you like most about being a footballer? Take our fun quiz to get news to suit your interests