Guilfoyle, from Royal Cork sailed ten points clear of Kinsale sailor O'Regan by the end of the 11-race series, with another Kinsale sailor, Aodh Kennedy, a further ten points behind in third.
Lizzy McDowell of Malahide took the senior silver fleet by some margin, finishing 27th in a 77-boat fleet studded with international competitors.
Peter McCann of Royal Cork brought more glory to Crosshaven, topping the junior fleet with Howth's Aoife Hopkins claiming top prize in the silver fleet.
Hilary Herlihy of Lough Derg YC won the regatta fleet.
Full results from the event are available through the regatta homepage, www.rsgyc.ie/optimist_nationals
A gallery of photos from the event is available courtesy of Fotosail.com