Borg in Mýrar was the site where Skalla-Grim, the father of Egill the hero of Egil's Saga settled in 891.
According to the saga, Skalla-Grímr Kveld-Úlfsson sailed to Iceland from Norway. His father, Kveld-Úlfr followed in the convey. Kveld-Úlfr became ill and orders a companion on his ship "You shall bear my greetings to my son Grímr, when you find each other, and tell him this too, if it happens that he comes to Iceland, and if it turns out that -- although it seems unlikely -- I am there ahead of him, then he shall settle in the place next to where I have come to land" (Egils saga Skalla-Grímssonar, ch 27)
Kveld-Úlfr died and, following his instructions, his coffin was cast into the sea. The coffin was washed into Borgarfjörður, the place where it washed up is called Kveldúlfshöfði ('Kveldúlfr's headland').
Therefor, Skalla-Grim built a house at Borg. The Saga tells us: "there was a great expanse of moor-land and it was widely-forested, a great distance between the mountains and the shore, plentiful seal-hunting and great stores of fish ... they found many ducks there [in one place] and called it Andakíll ('Duck-inlet') ... then then came to a certain small headland and hunted some swans there and called it Álptanes ('Swan-headland')" (ibid, ch 27).
The site now houses a church, graveyard and farm. The photo below is looking out from Borg towards the Borgarfjörð.