A nice example was found in Geir Zoëga's Dictionary here:
http://www.midhnottsol.org/zoega/h133.html

Illu feginn ver þú aldrigi !
»never rejoice in mischief«

It is nice because it illustrates the same point as with 'litlu'.
The literal translation is

[Over] [a] bad [thing] glad be thou never.

Here the adjective "bad" (=Illr) has been used in the
neuter singular Dative case. It is neuter because "thing"
is neuter; no gender can be assigned to an unspecified thing.
It is Dative because a preposition of the type "over" in
English tends to take the Dative, although the latter point
might perhaps be worth a further discussion (why it isn't
an Accusative).

Note also that "Illr" has here been used more or less as
a noun: bad things = badness = mischief.

Other examples of such usage op "illr" is found in Sigrdrifumál
no.'s 32 and 37:

32. Þat reþ ec þer it átta, at þv scalt viþ illo siá
oc firraz flerþarstafi; mey þv teygiat
ne mannz cono, ne eggia ofgamans.

37. Þat reþ ec þer it ellipta, at þv viþ illo siáir
hvern veg at vini; langt lif þicciomcac
lofþvngs vita, ra/mm ero róg of risin.


Best,
Xigung