According to several sources, the name of this waterfall is actually "Goðafoss", not "Hjöllsfjall". One argument which supports those sources is the fact that "foss" means "waterfall" in Icelandic, while "fjall" means "mountain". Here are some sources: 

https://www.europeanwaterfalls.com/waterfalls/godafoss-bjarnarfjordur/

The Dutch Wikipedia: https://nl.wikipedia.org/wiki/Go%C3%B0afoss_(Westfjorden)

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