You just park halfway along the street in Fintry (you cant get lost Fintry only has one street). There is a footpath which takes you up. Only ten minutes from the road with a bit of a steep scramble at the end. There are actually two forts up there - Dunmore (Big Dun) and Dunbeg (Little Dun). Dunbeg is in the woods on the way up. Dunmore is at the top. Years since I've been - my brother was a locum GP next door in Balfron way back in the day.