Don't take the quotes and info word for word, but rather look for the general themes involved. Bear with me.
I'll start with the Abyss. In many mythologies, the abyss or void represents a chaotic, primal world, particularly in Norse mythology. What I am theorizing here is that the Abyss of DS is the remnant(s) of this age, which Gwyn and company ended after defeating the dragons (creatures of the Abyss?) and starting the age of fire.
Now, on to the good stuff, Celtic mythology.
In Irish mythology, the Fomoire (or Fomorians) are a semi-divine race said to have inhabited Ireland in ancient times. They may have once been believed to be the beings who preceded the gods, similar to the Greek Titans. It has been suggested that they represent the gods of chaos and wild nature, as opposed to the Tuatha Dé Danann who represent the gods of human civilization.
Fomorians= Abyssal gods and/or primordial serpents
Tuathe De Danann= Gwyn and Co.
A group of people try to invade the land, but are oppressed by the Fomorians and leave, with very few survivors. Later, a group called the Fir Bolg go to the land, before the arrival of the Tuathe De Danann.
Next, the Tuatha Dé Danann, who are usually supposed to have been the gods of the Goidelic Irish, defeated the Fir Bolg in the first Battle of Magh Tuiredh and took possession of Ireland. Because their king, Nuada, had lost an arm in the battle and was no longer physically whole...(extraneous)
So the human gods took over, with the help of their king Nuada who is very, very interesting in regard to this theory.
After his arm is injured at the first battle of Magh Tuiredh, a new, half-Fomorian guy (relationships between the humans and Abyssal deities/creatures, may be important later) named Bres takes his place as king. Nuada is restored to the throne later, which starts the second battle, as the Fomorians want it back.
Bres, aided by the Fomorian Balor of the Evil Eye, attempted to retake the kingship by force, and war and continued oppression followed. When the youthful and vigorous Lugh joined Nuada's court, the king realised the multi-talented youth could lead the Tuatha Dé against the Fomorians, and stood down in his favour. The second Battle of Mag Tuired followed. Nuada was killed and beheaded in battle by Balor, but Lugh avenged him by killing Balor and led the Tuatha Dé to victory.
Nuada's great sword was one of the Four Treasures of the Tuatha Dé Danann, brought from one of their four great cities.
Balor was notable for his eye in the middle of his forehead and one directly opposite at the back, which meant he couldn't be sneaked up on from behind. According to prophecy, Balor was to be killed by his grandson. To avoid his fate, he locked his daughter, Ethlinn, in a tower made of crystal to keep her from becoming pregnant. However, Cian, one of the Tuatha Dé Danann, managed to enter the tower with the help of the druidess Birog. Ethlinn gave birth to triplets by him, but Balor threw them into the ocean. Birog saved one, Lugh, and gave him to Manannan mac Lir, who became his foster father. He was called Lugh Lamhfada and became a member of the Tuatha Dé Danann.
Lugh led the Tuatha in the second Battle of Magh Tuiredh against the Fomorians. Ogma disarmed Balor during this battle, but Balor killed Nuada with his eye. Lugh shot a sling-stone, which drove Balor's eye out the back of his head, where it continued to wreak its deadly power on the Fomorian army. In other versions, Lugh blinded Balor with a spear made by Goibniu or decapitated him and used his eye against the Fomorians .
One legend tells that, when Balor was slain by Lugh, Balor's eye was still open when he fell face first into the ground. Thus his deadly eye beam burned a hole into the earth. Long after, the hole filled with water and became a lake which is now known as Loch na Súl, or "Lake of the Eye", which is to be found in County Sligo.
With its foul distorted glances, its piercing beams, and venomous properties, like those of the basilisk this eye in the back of the skull would strike people dead. Because of this Balor kept it constantly covered except when he wished to get the better of his enemies.
An evil eye with basilisk-like qualities...Recall that the Ring of the Evil Eye is found on a body in the basilisk area of the depths. If this is truly the case, and we do encounter this monster in the DLC, better bring some Purging Stones along.
While we're on this note, here is an interesting tidbit:
In European bestiaries and legends, a basilisk is a legendary reptile reputed to be king of serpents and said to have the power to cause death with a single glance.
Could the Darklurker be the king/leader/originator of the primordial serpents, based loosely off the legend of Balor of the Evil Eye? Taking the above into account, as well as the footage of his arm grabbing and pulling you into the Abyss (almost primordial serpent style), I think it is somewhat of a compelling case. Or maybe I need to take off the tinfoil hat, I'm not sure.
Perhaps Nuada's status as king of Britain/knight of the Tuathe and his wounded arm might be one inspiration for Artorias and the events surrounding the DLC. Also, Balor may be the inspiration for the Darklurker or related Abyssal creature. The hero destroys him, then uses his eye to fight other demons, etc. I also think the obvious relationship between the pre-Tuathe humans and Fomorians is interesting. I'm thinking Oolacile and the Abyss here, though I found some other possibles within Norse mythology that I'll post later. Perhaps the humans had been working with or seeking out help from the Abyss (black magic, the witches, all that junk) to rebel against Gwyn.
All speculation and very much incomplete, just wanted to post this rough thought so I would remember it later and expand on it.
EDIT: I have cut out all the misc. stuff I found in order to keep this post shorter. I'll post it later if anyone is interested.