Hi, Erich!

Taken as a whole your translation is quite good. I would wish for
more consistency in your treatment of the names; sometimes you change
ON characters to English ones, sometimes you don't. And sometimes you
take the nominative form and sometimes the accusative. A minor quibble.

> Þat var á dögum Haralds konungs ins hárfagra,
> This was in the days of King Harald the Fairhaired,

Yeah. Personally I'd prefer "It was".

> Hálfdanar sonar ins svarta,
> son of Halfdane the black,

Aha.

> Guðrøðar sonar veiðikonungs,
> son of Guðrod, the hunting-king,

That's right.

> Hálfdanar sonar ins milda ok ins matarilla,
> son of Halfdane the merciful and the food-stingy,

Yeah. Well. I think 'mildr' means "generous" here.
As I remember it he was generous with gold and stingy with food :)

> Eysteins sonar freys,
> son of Eystein of Freyr

Lower case 'freyr'?

> Oláfs sonar trételgju Svíakonungs,
> son of Olaf treecutter, king of the Swedes,

Yup.

> at sá maðr kom skipi sínu til Íslands í Breiðdal, er Hallfreðr hét.
> that this man came by his own ship to Iceland to Breiðdal,
> who was called Hallfreð

Correct. (And awkard :)

> Þat er fyrir neðan Fljótsdalsherað.
> which is below the district Fljótsdal

Yes.

> Þar var á skipi kona hans ok sonr, er Hrafnkell hét.
> There on ship was his wife and son, who was called Hrafnkel.

Correct.

> Hann var fimmtán vetra gamall, mannvænn ok gørviligr.
> He was fifteen winters old, promising and enterprising.

I like 'promising' for "mannvænn".

> Hallfreðr setti bú saman.
> Hallfreð put together a farm.

You really say that? :)

> Um vetrinn andaðisk útlend ambátt, er Arnþrúðr hét,
> during winter a foreign handmaid died, who was called Arnþruð,

Handmaid? Slave?

> ok því heitir þat síðan á Arnþrúðarstöðum.
> and so since then they called it Arnþrudstad.

I'd prefer the passive voice: "It is called..."

> En um várit foerði Hallfreðr bú sitt norðr yfir heiði ok gerði bú þar,
> and in the spring Hallfreð brought his farm north over a heath
> and built a farm there,

Correct.

> sem heitir í Geitdal.
> where he called it Geitdal. (goat-dale)

Which he called?

> Ok eina nótt dreymði hann, at maðr kom at honum ok mælti:
> And one night he dreamed, that a man came to him and said:

Right.

> 'Þar liggr þú, Hallfreðr, ok heldr óvarliga.
> 'There you lie, Hallfreð, and rather incautiously,

Correct.

> Foer þú á brott bú þitt ok vestr yfir Lagarfljót.
> Bring away (by road) your farm and west over Lagarfljot.

Does 'á brott' especially carry the sense "by road"?
I suppose it's some sort of corruption of 'braut' but
I've never thought of it as having a sense related to that.

> Þar er heill þín öll.'
> There is luck for all thine.'

Hmm... That would be "Þar er heill þínum öllum".
This is nominative and has to mean: "There is all your luck."

> Eptir þat vaknar hann ok foerir bú sitt út yfir Rangá í Tungu,
> After that he woke and brought his farm out over Ranga in Tungu,

Right. It's very weird to see 'Tungu' in the translation, though.

> þar sem síðan heitir á Hallfreðarstöðum,
> where he afterwards called it Hallfreðstad.

Here I'd definitely prefer the passive voice.

> ok bjó þar til elli.
> and lived there to an old age.

Right.

> En honum varð þar eptir geit ok hafr.
> but he left behind him a nanny- and a billy-goat.

See below.

> Ok inn sama dag, sem Hallfreðr var í brott,
> and the same day, when Hallfreð was on the road,

Okay.

> hljóp skriða á húsin,
> a landslide fell down on the house,

Right.

> ok týndusk þar þessir gripir,
> and there perished these animals,

Slightly more literally: "and those animals were lost".

> ok því heitir þat síðan í Geitdal.
> and so he named that afterwards Goat-dale.

Once more I'd like the passive voice.

> A couple questions:
> 'Þat var...': is this just an existential construction, like English
> 'There was' ? I keep wanting to translate it literally 'This was in the
> days...', but it sounds false.

I think you have to rely on your own taste here.

> 'honum varð þar eptir': Gordon glosses this as 'he left behind', but I
> can't quite make sense of this phrase; 'it happened to him where after'?
> varð is from varða, 'to happen, to become'?

Here we obviously have a phrasal verb: "verða eptir". The best way to find
the meaning of such phrases is usually to look up the verb. The last usage
case of the verb given is usually what prepositions, in alphabetical order,
can accompany it and with what alteration of meaning.

Try here:

http://www.northvegr.org/zoega/483.php

In a stroke of luck this very phrase from Hrafnkels saga is given as an
example in the dictionary.

Kveðja,
Haukur