Eldborg is a volcanic crater in Western Iceland. Is it possible to reach the crater by public transportation? Or what is the closest one can get using public transportation (probably bus)?
What you probably want to search, is the route from Reykjavík to Stykkishólmur. You will have to switch buses at Borgarnes (from no. 57 to no. 58). Make sure the driver will drop you off where you want though. You can also try hitchhiking from Borgarnes. In the summer there are a lot of tourists and Snaefellsnes peninsula is quite popular destination.
Also, make sure to be dropped off at the road to the farm south of Eldborg (Snorrastadir) and take an "official" trail from there. As far as I recall, there is a fence around the crater area (it's nature's reserve I think) and the terrain around it is quite difficult. You can also sleep at this farm if need be - there is camping site, showers, kitchen etc.
EDIT: I highly recommend to use this instead of Google maps, it is just much better (you can see the trails etc.): Ja.is maps