I would say it depends on how it fits into your storyline. The setting and background I have been tasked with writing for another RPG- and the contract also stipulated a minimum of three novels should they option them- is based directly in existing ancient legends and lore. So, I have to do extremely detailed and extensive research into those pre-Christian religions and cultures that I am unfamiliar with (since the setting involves several European, African, Asian, Oriental, American- north and south- and Australian indigenous religions, and my expertise is on the European ones) and get it right. The last thing I want is a ton of nastygrams from thousands of angry Hellenes in Greece because I screwed up something basic about the Olympian Gods. If the core of the storyline is not based in a specific mythology or requires precise accuracy, get creative. A good example of a blend of legends throughout history that has been "tweaked" with significant creative license is Feist's Fairy Tale.
A second point to remember- and I touched on this a bit above- is that a lot of those ancient religions and their attendant mythologies are STILL practiced today, and many of the followers of those ancient faiths- or their derivatives- are likely to be in your target audience. Granted, not all of them are thin-skinned purists who will show up on your doorstep if you bugger up a deity's name. Hell, I went to see Thor with a couple of Theodisc (Norse/Germanic heathens to whom Thor IS one of their Gods) fully expecting them to burn the theater to the ground and blood-eagle somebody. I was surprised, they loved it (granted, largely because it showed their Gods in a pretty positive light, and never specifically said they weren't Gods), so if you get the gist right and are flattering or fair, they will likely let the details slide. I know a lot of Ásatrú and Theodisc Dresden fans who actually cheered the portrayal of Valkyries and Odin in the last two novels
has introduced the Fomóirí into the mix, I am a bit on pins and needles to see how he will treat the Gods of my people, the Túatha de Dannan, since the Fomóirí are their ancient enemies (when they weren't knocking each other up, at least with the pretty ones out of the Fomóirí) and a core part of our faith (Sinnsreachd).
My $0.02 anyway :D
Those are daoine sídhe, though, not Gods. The Túatha de Dannan consists of between twelve and thirty primary and secondary deities, including Nuada Lámhairgid (king of the Túatha, alternating with Lugh periodically), Lugh Lámhfada, the Ildánach or many-skilled, Danú, the mother-figure of the Tútha de Dannan and, among other aspects, the Goddess of motherhood, Eochaid Ollathair or "an Dagda", a wise and boisterous God of masculinity who revels in feasting, fighting, acts of strength, harping, chasing women, etc., Brighíd the Goddess of poetry, fire, smithing, and inspiration of musicians and bards, Áine the Beautiful, an Mórríghan, "the Great Queen" who rules over the battlefield and governs prophecy, Goibhniu the smith, Creidhne the artificer, Macha, the Goddess of horses, war, sovereignty, and the sacral bond between a rí (king) and the land, Oghma the god of knowledge, warrior-poet natures, and eloquence, Manannán Mac Lír the Waverider, God of the seas and guardian of the gateways between this world and the otherworlds, Badb Catha the Lady of Victory and contemporary of the Brythonic Andraste, Dian Cécht the physician of the Gods, Cromm Cruach the Old Man of the Hill, and many other regional Gods and Goddesses such as Bóann who governs the rivers and waters inland, etc.
None of these have yet been introduced or even mentioned, only the Túatha as a whole in passing in Ghost Story.
The Túatha de Dannan were relegated to "fairies" by Christian missionaries who met with stiff resistance in their conversion efforts in Ireland. In order to ease the transition, they first put the Christian God in as the king of the Gods, and then- over time- reduced the Irish Gods to fairies to allow the Gael to continue venerating them without impinging upon Christian conversions. This created a unique blend of foreign Abrahamic beliefs- which originate in the Middle East- and indigenous Irish beliefs that culminated in what is referred to as the Celtic Church. It was this "heretical" faith that in the 12th century caused Pope Adrian IV- using the so-called Donation of Constantine (a document purporting to be written by Emperor Constantine giving complete control over the Empire and it's lands to the Pope, a document later proven to be an 8th century forgery by Pope Stephen II or someone within his papal court used in a power-grab against the Franks- to give Ireland to King Henry II in exchange for the Angevin king forcing Ireland to come in line with the Roman church. The adherence to "pagan" traditions and elements was unacceptable to the Roman authorities as they sought to consolidate and homogenize the Church, and the continued veneration of the Túatha de Dannan- even in a degenerated and watered-down form- smacked of pagan idolatry to the Pope. So, the veneration of the Túatha de Dannan underwent even further degeneration into folk customs and traditions in which 99% of the Irish forgot the origins of who they were venerating. Only a few instances- most of which involved Badb Catha or Áine- occur in later years wherein they are remembered as deities rather than fairies. The most notable was Badb Catha's appearance before the Battle of Dysert O'Dea, where she granted Clan Turlough victory. After the battle she was honoured in the Caithréim Thoirdhealbhaigh, or The Triumphs of Turlough- as "a war-goddess woman-friend".
It was only in the late 19th century with the works of Lady Augusta Greggory and W.B. Yeats that the Túatha de Dannan were once again written in literature (Greggory's Gods and Fighting Men most notably) as Gods and not fairies. That was the beginning of a resurgence of indigenous Gaelic faith that still continues to this day. Many folks out there pray to the Túatha de Dannan and follow the faith and customs of their Ancestors, just as many worship and honour the Æsir and Vanir, the Olympians, the Häme-Karjalaso Gods, the Gods of the many American peoples, etc. As a result, it is important that- as authors- we get it right if we wish to avoid the upset or outright angry emails and rantings :)