Took a while to find this place, not least because Google Maps shows it down the wrong end of Skolavordustigur, and also because it's not actually /on/ Skolavordustigur, despite the address.
If you've got a proper map (like Osmand) it's at 64.14586N 21.93282W. Which is the first time I've ever given a lat-long reference for a restaurant. You can get to it off Ingolfsstraeti by going to the junction with Amtmannsstigur then turn your back on Amtmannsstigur and walk up the parking area and up the steps you'll see in front. You can also get to it from Bergstadastraeti, it's to the right of Bergstadir car park.
"Quirky", probably best description. And not just because it takes a geography degree to find it. Odd decoration, confused staff, a general air of idealism. However.... the food was delicious and the plates full. There was a regular stream of local residents in for the take-away which is always a good sign. Essentially, it's an old-fashioned veggie restaurant where the main thing is the food, and they do that bit very well.
If you own or manage Graenn Kostur, register now for free tools to enhance your listing, attract new reviews, and respond to reviewers.