Stafr lifir einn þars inni
unnfúrs viðir brunnu
(synir ollu því snjallir
Sigfúss) Níals húsa ;

"One post only lives/remains where timbers of wave-fire [=timbers of
gold = men] burned in Njal's house. The brave sons of Sigfus caused
that."

The compound kenning `viðr unnfúrs' = wood of wave-fire = wood of gold
= man. It follows a very common pattern of kenning formation, where
gold is called the fire of any kind of water, and men called any kind
of tree or wood, but is, of course, especially appropriate here. The
allusion is to the feast which Ægir, the giant ruler of the sea, held
for the gods, which was lit by the brightness of his gold
(Skáldskaparmál, chapter 41) [
http://www.heimskringla.no/original/snorre/skaldskaparmal.php ].

`húsa' is plural (collectively the buildings that make up the
farmhouse), genitive following `inni' "inside (of)".

nú hefr Goldnis sonr goldit,
gekk eldr of sjöt rekka,
ljóss brann hyrr í húsum,
Höskulds bana ens röskva.

"Now, Gollnir's son, fire went over the warriors' abode. Bright
burned fire in the buildings."

Nominative `ljós' and `hyr' in our version are, as Alan correctly
suggested, Modern Icelandic spellings of `ljóss' and `hyrr'. The most
famous `rekkar' "warriors, heroes" of Old Norse legend were Hálfs
rekkar, who were also burnt inside a hall. They're mentioned by Snorri
in his discussion of peotic terms for men/warriors, and their story is
told in the saga of King Hálfr.

Mundit mellu kindar
miðjungs brúar Iðja
Gunnr of geira sennu
galdrs bráregni halda,
es ...

"The woman would not hold back her brow-rain [=tears] at the [thought
of the] quarrel of spears [=battle] of the
god-of-bridge-of-offspring-of-ogress [=god of bridge of giant = god of
shield = warrior] when..."

`mundit' = `mundi' "would" + the archaic negative suffix –t. Thus
`mundit' = "would not".

`miðjungr brúar kindar mellu' "god of bridge of offspring of ogress" =
god of bridge of giant = god of shield = warrior. This kenning alludes
to the myth of the giant Hrungnir who was tricked into standing on his
shield in the mistaken belief that Thor was burrowing underground to
attack him from below (Skáldskaparmál, chapters 24 and 61).

`Iðja galdr' "giant's spell" = gold. Kennings which refer to gold as
the speech or mouthful of giants allude to the story of how Thjazi's
sons shared out his gold among themselves after his death. Each got to
keep as much gold as he could fit in his mouth (Skáldskaparmál, ch. 4).

`senna geira' = "quarrel of spears", "abusive speech of spears" =
battle. Another traditional kenning type.

`Gunnr' is the name of a valkyrie, used conventionally in poetry like
the names of other supernatural females to mean `woman'.

hræstykkis hlakka
(hraustr) síns vinar mínu
(tryggvik óð) ok, eggjar,
undgengin, spor dunðu.

The second half of this strophe was considered garbled by Finnur
Jónsson and incomprehensible. Magnus Magnusson and Hermann Pálsson
also have a note to this effect in their translation. `eggjar spor'
"track(s) of blade" means "wound(s)".