Dúrnir was a dwarf who appears in three Old Norse skaldic poems, which suggests that he once was a well-known dwarf in Norse mythology.
The most notable poem is
þá er í stein
ept dvergi hljóp,
ok salr bjartr
við jöfri gein.
By Diurnir's elfin race,
Who haunt the cliffs and shun day's face,
Swegde was deceived, The elf's false words the king believed.
The dauntless hero rushing on,
Passed through the yawning mouth of stone:
It yawned – it shut – the hero fell,
Saekmime's hall, where giants dwell.  
A more literal translation:
spawn of Durnir
warden of the hall
who into stone
the rash hero
ran after the dwarf.
The bright hall
built of giants
by the chieftain`s presence.
He also appears in a list of Dwarves in the anonymous
aurvangr ok dúfr,
ónn ok draupnir,
dori ok dagfinnr,
dulinn ok ónarr,
alfr ok dellingr,
óinn ok durnir.
The third poem is found in
Kveða skal hróðr fyr hríðar
hræ-blakks viðum sævar,
drykkr var Durnis rekkum
døkkr, ljósara nøkkvi.
Snorri also includes Dúrnir in a list of giants in the section of his Skáldskaparmál (Faulkes translation, p. 157). Prose Edda
It is possible that the name Durnir is an
emendation (or a case variety) of Durinn, mentioned as the father of dvarfs in Dvergatal. Both names mean door, or door-warden. The names Durinn og Durnir do not appear in the same texts. The Norwegian translation of Ynglinga Saga from 1900 uses the name of Durinn instead of Durnir.
^ Ynglinga saga at Norrøne Tekster og Kvad
^ A second online presentation of Ynglingatal
^ Laing's translation at the Internet Sacred Text Archive
^ Laing's translation at Northvegr
^ Skaldic Poetry of the Scandinavian Middle Ages
^ At Skaldic Poetry of the Scandinavian Middle Ages