Thanks, Alan, for sorting out the bits I didn't get.
Grace
sögðust kenna að vísu "og erum vér komnir við Orkneyjar í Hrossey."
before and (the) said-themselves to recognise certainly (at vísu, under
víss, Z4) 'and we are (have) come (arrived) by (the) Orkney Isles in Hrossey
(Horse Island).'
Jarl hafði spurt áður brennuna og kenndist hann af því þegar við mennina.
(The) earl had learned before of the burning and he recognised (see kennast
við, Z12) the men from that at-once.