Dtyjlui

Since Tolkien was largely influenced by the Old English poem Beowulf, it should not be a surprise if his Orcs were at least partly derived from it.

Just as the Orcs are twisted, monstrous, naturally evil descendants of Elves, so is Grendel a twisted, monstrous, naturally evil descendant of Cain, and thus Adam. There is a similar motif of mundane beings being corrupted into monsters.

Furthermore, one descendent of Cain mentioned in the poem is referred to to as an orcneas.

Unlikely, since Tolkien was professor of Old English (and as Valorum commented, wrote a translation and commentary on Beowulf). The Old English word "orc" corresponds with Latin Orcus (deity of the Underworld), and synonymous with the Norse þyrs/ðyrs "ogre", so it has a different mythological origin than Grendel, and Tolkien would certainly have been aware of that.

Grendel is referred to in Beowulf as a sceadugenga – shadow-walker, night-goer. Orcs are mentioned apart from Grendel in lines 111–114 in Beowulf:

Þanon untydras ealle onƿocon

eotenas ond ylfe ond orcneas

sƿylce gigantas þa ƿið gode ƿunnon

lange þrage he him ðæs lean forgeald

Thence all evil broods were born,

ogres and elves and orcs

—the giants also, who long time fought with God,

for which he gave them their reward

Tolkien's Orc origins are first described in The Tale of Tinúviel as "foul broodlings of Melkor who fared abroad doing his evil work".

See also: Origins of Orcs in Middle-Earth

To quote from Tolkien's "Commentary accompanying the translation of Beowulf", published by Christopher Tolkien and based on Tolkien senior's lecture notes from the 1930s (as the Professor of Anglo Saxon at Oxford, per Codosaur's answer):

90–1 haunting shapes of hell; *112 orcnéas

The O.E. word occurs only here. orc is found glossing Latin Orcus [Hell, Death]. neas seems certainly to be né-as, plural of the old (poetic) word né 'dead body'. This appears also in né-fugol 'carrion bird'. Its original stem in Germanic was nawi-s: Gothic naus (plural naweis), Old Norse ná-r.

'Necromancy' will suggest something of the horrible associations of this word. I think that what is here meant is that terrible northern imagination to which I have ventured to give the name 'barrow-wights'. The 'undead'. Those dreadful creatures that inhabit tombs and mounds. They are not living: they have left humanity, but they are 'undead'. With superhuman strength and malice they can strangle men and rend them. Glámr in the story of Grettir the Strong is a well-known example. (Tolkien, Beowulf, A Translation and Commentary pp 163–4)

Based on these associations that Tolkien gives to orcnéas (at least at the time when this commentary was written), especially the explicit suggestion of barrow-wights, I think it is safe to say that while the word orcnéas, or even the latin orcus may have been a source for Tolkien's 'orc' (which he emphasises is not associated with Orca), the orcnéas as creatures are not themselves the prototypes for his Legendarium orcs.