Said to be a ring barrow, I guess that's as good a name as any. Herity says that "The form of this site is similar to that described in the OS Letters as the 'Rath of Cumhal", giving the nearby town its name, Rathcoole (old Fionn again).
It's on the southern slope of Windmill Hill, about 300 metres down from the road. The overall diameter is 80 metres, with the internal flattened mound about 30 metres across. Both fosses are better preserved on the south-eastern quadrant, the two almost a metre and a half deep.
I've seen double- (and triple-) ramparted raths before, but never a double-ditched barrow. It's a thing of structural beauty, slightly spoiled by the nearby pylon and truncated on its western arc.
I had parked badly here and may have come the attention of the locals so I was a tad uncomfortable and didn't stick around to fully appreciate this site. Still, another Dublin (the Kildare border is about 200 metres away) monument that I'd only heard about very recently – and one of substance at that.