Tröllafossar or Tröllfossar is a very beautiful waterfall in the river Leirvogsá, not far from Reykjavik in the west of Iceland.
From Reykjavik take road 1>36 direction Thingvellir. Just for Thingvellir turn left to Stardalur. Keep left: the road end on a small path following the river. I am not sure if this road is suitable for 2WD. Otherwise it is possible to walk the last part of the road.
Another way is a 1,5 kilometer walk up by the river from the farm Skeggjastaðir along road 4365 to Hrafnholar.