This waterfall can be reached in two ways: the long way or the lazy way. You can walk for an hour, following the river from Gljúfrasteinn (the house of author Halldór Laxness), or drive a little further up the Mosfellsdalur valley, turn up a side road, park at the end of it and walk for 10 minutes.
The waterfall is believed to have been named after a woman named Helga, who was the daughter of Bárður Snæfellsás. An alternative theory is that “helga” comes from “helgi”, i.e. “holy” and it simply means “the holy waterfall” because it used to belong to the Mosfell church. Whatever the provenance of the name, the waterfall is lovely and there is a nice picnic spot nearby. I think people even bathe in the pool beneath the fall, because I once found a pair of swimming trunks on the riverbank.