Faerie has been a lot of different things over the various editions. In the adventure "Legacy of the Lisosalfar" in Dungeon #42 it was considered to be a parallel Material Plane. In published Greyhawk the closest thing is the Fading Lands detailed in From the Ashes
, which include the Faerie Court of Rings, a Stonehenge-like capital of the Seelie Court accessible through a portal in the Welkwood.
This is complicated somewhat by the Seelie Court described in Monster Mythology
(which Carl Sargent wrote at around the same time as he wrote From the Ashes
; the two books share a lot of references), which is maybe identical to the Faerie Court of Rings even though it's said to move through the outer planes of good and chaos. The Unseelie Court, its wicked counterpart, is in Pandemonium (according to Monster Mythology
The Fading Lands are defined as demiplanes coexistent with the Prime Material Plane and associated with specific geographical areas there. It wouldn't be difficult at all to redefine them as something like 4e and 5e's Feywild, though, a parallel plane where Oerth's geographic features have Feywild equivalents. So the Faerie Court of Rings exists in the Feywild version of the Welkwood, and the Mines of Dumathoin in the Feywild version of the Stark Mounds. I'd probably move the Crypt of Iron Souls and the Blood Obelisk of Hextor to the Plane of Shadow, though (the former paralleling the Glorioles; the latter paralleling the Gull Cliffs).
Another thing to consider is the Kingdom of Caer Sidi in Q1, which is based on (ripped off from) the Faerie in Poul Anderson's novel Three Hearts and Three Lions
. This could be part of the same Feywild as the Seelie Court, but it might be the Feywild of another world.
Still another reference to maybe include is UK1 Beyond the Crystal Cave
, which details an otherworldly garden paradise populated with leprechauns, dryads, and other fey creatures. Porpherio's garden could exist in the Feywild version of Sybarate Island, especially considering the oddly flowing time effect.
Finally, consider making EX1 and EX2 part of the Feywild (and even the Isle of the Ape).
For what it's worth, Paul Kidd's Greyhawk novels mentioned the plane of Faerie by that name (one of the protagonists, Escalla
, is a literal fairy princess).
As Havard mentioned, PC1 Tall Tales of the Wee Folk
was a basic D&D (BECMI) accessory with rules for creating PC versions of the various sylvan races.
Bastion Press's d20 book Faeries
is one of the better takes on a Plane of Faerie, written to be very compatible with extant D&D lore. It also has a Stonehenge-like circle in the center of the faerie realm which could be assumed to be identical with Greyhawk's Court of Rings.
Beyond Countless Doorways
from Malhavoc Press included a faerie plane called Faraenyl. It appears to be a standard fey land, with oddly flowing time and realms divided by season, but there's a major twist that means it's not exactly what it seems and its inhabitants play a stiff price for maintaining it.