I found a description of Icelandic haystacks that’s very
helpful for understanding this bit. My translation from
Emil Dagobert Schoenfeld, Das Pferd im Dienste des Isländers
zur Saga-Zeit: Eine kulturhistorische Studie, Hamburg, 2011,
pp. 9-10:

The hay itself was stored in two ways, either in a barn
<hlaða> or in a free-standing haystack, called a
stakkgarðr or heygarðr when it was square in shape and a
heyhjálmr (hay-helm) when conical in shape. The latter
was the preferred form for those haystacks that were left
standing in the meadows by the <sel>. But in all cases
the haystacks were surrounded with peat sods for
protection against the penetrating winter damp: on the
sides with compact cubical pieces, on top with thin peat
flaps cut in long strips.

This explains why I’ve chosen to translate <garðr> as
‘haystack’ rather than as ‘wall’.

> En er þeir Snorri komu að garðinum þá er eigi getið að
> þeir hefðust orð við og veittu þeir honum þegar atgöngu og
> mest með spjótalögum en Arnkell laust af sér með meiðnum
> og gengu mjög í sundur spjótsköftin fyrir þeim en Arnkell
> varð eigi sár.

> When they, Snorri (and the others), came to the stronghold
> where it is not spoken (?) that they had spoken and they
> attacked him at once and most(ly) with spear thrusts, but
> Anrkell himself struck with injuries and spear shafts went
> asunder before them, but Arnkell didn’t become injured.

> And when they, Snorri (and co.), came to the yard then is
> not mentioned that they had words with (Arnkell) and they
> immediately gave him an attack and most with spear
> thrusts, but Arnkell struck (the thrusts) away from
> himself with the runner and many spear shafts went asunder
> for them, but Arnkell was not injured.

And when Snorri and his folk came to the haystack, it is not
mentioned that they [presumbly Snorri and Arnkel] spoke with
each another, and they attacked him immediately, and mostly
with spear-thrusts, but Arnkel warded [them] off with the
runner, and the spear shafts for the most part broke apart
before them, and Arnkel was not wounded.

Here <mjök> is 'for the most part, to a great extent'.

> En er þeir höfðu látið skotvopnin þá rann Þorleifur kimbi
> að garðinum og hljóp upp á garðinn með brugðið sverð en
> Arnkell laust sleðmeiðnum í mót honum og lét Þorleifur þá
> fallast undan högginu öfugur út af garðinum en meiðurinn
> koma á garðinn og gekk úr garðinum upp fyrir jarðartorfa
> frosin en sleðmeiðurinn brotnaði í fjötraraufinni og hraut
> annar hluturinn út af garðinum.

> And when they had given up the missiles, then Thorleifr
> Kimbi ran to the stronghold and jumped up to the
> stronghold with a drawn sword, but Arnkell struck with the
> sledge runner against him and caused Thoreliefr then to
> fall backwards from the blow out of the stronghold and
> came injured to the stronghold and went out the stronghold
> up against frozen ground-turf and the sledge runner broke
> in the hole in the sledge runner and flew in pieces out of
> the stronghold.

> And when they had loosed? missiles then Thorleif kimbi ran
> to the yard and leaped up on the wall with a drawn sword,
> but Arnkell struck towards him with the sled runner and
> Thorleif then turned the blow? the wrong way let himself
> fall from below? from the the wall and the runner came on
> the wall and went out of the wall up before the frozen
> wall turf and the sled runner broke at the hole where the
> strap goes and one part flew out from the wall.

And when they had lost [i.e., exhausted] the missile
weapons, Þorleif kimbi ran at the haystack and leaped up
onto the haystack with drawn sword, and Arnkel struck at him
with the sledge-runner, and Þorleif let himself fall
backwards away from the blow out from the haystack, and the
runner hit the haystack, and a frozen slice of sod went
forth up in front of the haystack, and the sledge-runner
broke at the hole for the peg by which it was attached to
the sledge, and one piece flew out of the haystack.

<Láta fallask> is 'to let oneself drop', and <undan> can be
'away from'. <Jarðartorfa>, which has <torfa> as its second
element, is nominative, so it must be the subject of <gekk>.

The edition that my local group used had this footnote:

Sleði sá, sem hér er talað um, hefur verið á þessa leið:
Sleðinn rann á meiðunum, sem voru bognir upp að framan;
lóðrétt upp af meiðnum stóðu trébútar, sem nefndust
fjötrar, en fjötrarauf holan, sem þeir gengu niður í. Ofan
á fjötrana voru festar langfjalir, sem líka voru negldar
við meiðinn að framan; við þær voru festar þverrimar
sleðans.

The sledge that is spoken of here was of this design: the
sledge ran on the runners, which were bent up in front;
wooden pegs called fjötrar stood vertically up from the
runner, and the hole into which they went down was called
the fjötrarauf. Planks were fastened down onto the pegs,
which similarly were nailed to the runner at the front; to
them were fastened the sledge’s cross-rails.

> Arnkell hafði reist við heyið sverð sitt og skjöld.

> Arnkell had raised (brought up?) with the hay his sword
> and shield.

> Arnkell had cut the hay with his sword and shield.

Arnkel had set up his sword and shield against the hay.

<Reist> is a past participle; the pp. of <rísa> is <risinn>,
without a <t>, so this must be <reisa> 'to raise, to set
up'. (The second gloss is from Faulkes & Barnes, the
glossary volume of A New Introduction to Old Norse.)

> Tók hann þá upp vopnin og varðist með þeim.

> He then took up his weapon and fought with them.

> Then he picked up the weapons and defended himself with
> them.

He then took up the weapons and defended himself with them.

> Varð honum þá skeinisamt.
> He then became exposed to being wounded.
> Then he was exposed to being wounded.

He was then exposed to being wounded.

> Komust þeir þá upp í garðinn að honum en Arnkell hljóp upp
> á heyið og varðist þaðan um hríð.

> They then got through to him up in the stronghold, but
> Arnekll jumped up on the hay and was there a while.

> Then they came up to the wall to him and Arnkell leaped up
> on the hay and defended from there for a time.

Then they made it up onto the haystack to him, but Arnkel
leaped up onto the hay and defended himself from there for a
while.

I’m not entirely sure how to picture this; my best guess is
that the hay is stacked significantly higher towards the
centre, and that Arnkel has retreated to that higher stack.

> En þó urðu þær málalyktir að Arnkell féll og huldu þeir
> hann í garðinum með heyi.

> And yet it (plural in ON) became the conclusion (plural in
> ON) that Arnkell fell (dead), and they covered him in the
> stronghold with hay.

> But still the outcome of those events is that Arnkell fell
> and they hid him in the yard with hay.

But nevertheless the end of the matter was that Arnkel fell,
and they covered him with hay in the haystack.

> Eftir þetta fóru þeir Snorri heim til Helgafells.

> After this, they, Snorri (and his allies), went home to
> Helgafell.

> After that they, Snorri (and co.) went home to Helgafell.

After that Snorri and his folk went home to Helgafell.

> Um dráp Arnkels kvað Þormóður Trefilsson vísu þessa:

> Concerning Arnkell’s slaying, Thormodr Trelifson recited
> this verse:

> Thormod Trefils’ son recited this verse about the killing:

Þormóð Trefilsson spoke this verse about Arnkel’s killing:

> Fékk hinn fólkrakki,
> He became a folk dog,
> Made the folk-dog??

> framdist ungr sigri,
> was furthered a young victory,
> advanced itself younger victory,

> Snorri sá-orra
> Snorri the moor-fowl
> Snorri that ??

> sverði nógs verðar.
> Enough meal for a sword.
> by a sword ?? ??

> Laust í lífs köstu
> Struck in life’s pile (??)
> Thrust to life by choice?

> Leifa máreifir
> Leave behind raven-(eifir) (??)
> abandon sea-?

> unda gjálfrs eldi
> dwelt the sea’s din kindled
> work of din of the sea by fire

> þá er hann Arnkel felldi.
> Where Arnkel fell.
> then when he, Arnkell, was felled.

This gets very interesting, because the various editions
don’t even agree on the exact wording of the vísa.
Fortunately, this version agrees (apart from spelling
conventions) with what I believe to be the most up modern of
the scholarly editions available to me:

Fekk enn folkrakki,
framðisk ungr sigri,
Snorri sár-orra
sverði gnógs verðar;
laust í lífs kǫstu
Leifa máreifir
unda gjalfrs eldi,
þás hann Arnketil felldi.

Folling the definite article, <folkrakki> is the nom. sing.
masc. in the weak declension of the adjective <folkrakkr>
'valiant, stouthearted', mentioned in CV s.v. <fólk>. This
is an instance of the poetic usage <folk> 'battle', so the
literal sense is something like 'battle-erect'. The
<sár-orra> 'wound-heathcock' is a raven. <Sverði> is an
instrumental dative, ‘with a sword’.

<Lífs kestir> 'life’s piles' (here in the accusative) refers
to the body, and more specifically to the torso, perhaps
even more specifically to the chest. The imagery isn’t very
clear. The most plausible explanation that I’ve found is
that the body is being likened to a pile of wood that serves
as fuel for a fire, perhaps even more specifically to a
funeral pyre: in some sense it is life’s fuel. I’ve adopted
that view in the attempt at a fairly literal translation
below.

<Reifir> is an agent noun from <reifa> 'to enrich', so
<máreifir> is 'sea-mew-enricher'. Leifi is the name of a
sea-king; his sea-mew is a raven, and the raven-enricher is
a warrior. <Unda> is the gen. plur. of <und> 'a wound'.
Zoëga’s gloss 'din of the sea' for <gjálfr> is a bit
inadequate; in kennings the word often seems to be simply
'sea', and I’ve also seen it translated 'surge' and 'wave'.
My impression is that it’s probably fairly safe to think of
it as 'sea', but with overtones of surging waves and the
noise associated with them. At any rate, <unda gjálfr>
'wounds’ sea(-surge)' or the like, is blood, and blood’s
fire is a sword.

Won, the battle-erect,
(distinguished himself young with victory)
Snorri, for wound-heathcock
with sword a plentiful meal;
struck in life’s (wood)piles
Leifi’s sea-mew-enricher
with fire of wounds’s sea(-surge)
when he felled Arnkel.

In prose word order:

Enn folkrakki Snorri fekk sár-orra sverði gnógs verðar;
framðisk ungr sigri; Leifa máreifir laust í lífs köstu
unda gjalfrs eldi, þás hann felldi Arnketil.

The valiant Snorri won with sword a plentiful meal for
raven; distinguished himself young with victory; [the]
warrior [= Snorri] struck in the breast when he felled
Arnkel.

Brian