This is by no means a complete list of Celtic Gods, I'm sure. What's here is all I've come across so far, if you'd like to find out more on the below deities, or wish to learn more in general, the library probably has good resources. 

Amaethon - Wales; God of agriculture.

Angus Mac Og - Ireland; God of youth, love, and beauty. One of the Tuatha De Danann, name means "young son". He had a harp that made irresistible music, and his kisses turned into birds that carried messages of love. His brugh, underground fairy palace, was on the banks of the Boyne River. Variants: Angus or Oengus of the Brugh, Angus Mac Oc.

Arawn - Wales; God of the dead and the underworld Annwn. Only until Christian conversion, the Welsh didn't look on the underworld as hell. God of revenge, terror, and the dead.

Bel - Ireland, Wales; God of cattle, crops, fertility, fire, healing, hot springs, prosperity, purification, science, success. A sun and fire god closely connected with the Druids and the festival of Beltaine (May 1). Variants: Belenus, Belinos, Beli Mawr (Wales).

Bran the Blessed - Wales; God of prophecy, the arts, war, music, writing. Associated with ravens; the brother of Manawydan ap Llyr and Branwen. His father Llyr was a sea god. Variant: Benedigeidfran.

Cernunnos - all Celtic areas in some form; God of animals, commerce, crossroads, fertility, reincarnation, virility, warriors, woodlands. Druids knew him as Hu Gadarn, the Honored God. Ancient Celtic images show him seated in a lotus position, naked, with antlers or horns on his head. Animals that were sacred to him: bull, ran, stag, and horned serpents. Variants: Cerowain, Cernenus, Herne the Hunter.

The Dagda - Ireland; God of the arts, knowledge, magic, music, prophecy, prosperity, regeneration. Known as the "Good God" and "Lord of the Heavens," he was one of the high kings of the Tuatha De Danann and had four great palaces under hollow hills. Of his children, the most important are Brigit, Angus, Midir, Ogma and Bodb the Red. His magical cauldron had an inexhaustible supply of food and his oak harp made the seasons change.

Diancecht - Ireland; God of healing, magic, medicine, regeneration. Physician-magician of the Tuatha De Danann; his sons were Miach, Cian, Cethe, and Cu, his daughter Airmed was also a great physician. Variant: Dian Cecht.

Dylan - Wales; sea deity and the some of Gwydion and Arianrhod, this God was called Son of the Waves, and a silver fish was his symbol.

Goibniu - Ireland, Wales; God of blacksmiths, weapon-makers, brewing. One of a triad of Tuatha De Danann craftsmen, he was called the Great Smith. Weapons that he forged always hit their mark and made fatal wounds. The other two craftsmen were Luchtain the wright, and Creidne the brazier.

Gwethyr - Wales; King of the Upper world, this God was the opposite of Gwynn ap Nudd.

Gwydion - Wales; God of enchantment, illusion, magic. A son of Donn, the sea goddess, and brother to Govannon, Arianrhod, and Amaethon (god of agriculture). Known as a great wizard and bard in northern Wales. He was many skilled, like the Irish god Lugh, he was a shapeshifter whose symbol was a white horse.

Gwynn ap Nudd - Wales; first known as King of the Fairies and Lord of the Underworld, this God later ruled over the Plant Annwn, subterranean fairies.

Llew Llaw Gyffes - Wales; son of Arianrhod and raised by his uncle Gwydion. A curse prohibited him from having and earthly wife, so his uncles made him one out of flowers and named her Blodeuwedd. She and her lover, Gronw Pebr, plotted Llew's death, but because of Llew's divine origins, the death simply became an annual duel between the two men. His symbol is a white stag, and is celebrated on August 1, the Celtic ceremony of Lunasa.

Llud Llaw Ereint - Wales; God of harpers, healing, poets, smiths, sorcerers, and waters.

Llyr - Ireland, Wales; God of sea and water, may have also ruled the underworld. The father of Manawydan, Bran the Blessed, and Branwen.

Lugh - (Loo) Ireland, Wales; a sun God of all crafts and arts, healing, journeys, prophecy. Son of Cian, a Tuatha De Danann. Of legend, his skills were without end; in Ireland he was associated with ravens; and a white stag as his symbol in Wales. He had a magic spear and otherworldly hounds. His festival was Lughnassadh, or Lunasa - August 1. Variants: Llew, Lug, Lugus, Lugh Lamhfada (of the long arm), Lug Samildananch (much skilled).

Manannan Mac Lir - (May-nah-naun) Ireland, Wales; a shapeshifting God of the sea, magic, navigators, commerce, storms, rebirth, weather. The chief Irish sea god whose special retreat was the Isle of Man. In Wales his name was Manawydan ap Llyr. He had several magical weapons and a suit of armor that made him invisible; and his swine kept the Tuatha De Danann from aging.

Math Mathonwy - Wales; legend has him as a king who was also a God of enchantment and magic.

Merlin - Wales, Britain; God of all forms of magic and prophecy, healing, illusion, the arts. Originally an ancient Welsh Druid, priest of the fair religion, and great magician. He was transformed in the later Arthurian sagas. Tradition says he learned his powerful magic from the Goddess in her forms of Morgan, Viviane, Nimue, and Lady of the Lake. Legend says he now lies sleeping in a hidden crystal cave. Variants: Merddin, Myrddin. More info on Merlin & Nimue

Neit - Ireland; God of battle.

Nuada - (Noo-ada) Ireland, Wales; God of harpers, healing, historians, magic, poets, warfare, writing. King of the Tuatha De Danann at one time, he had to step down when he lost his hand in battle; it was replaced by a silver one. Variants: Lud, Lludd, Llaw, Ereint, Nudd, Nodens.

Ogma - God of eloquence, inspiration, language, magic, music, physical strength, poets, writers. Invented the Ogam script alphabet and carried a huge club similar to Hercules'. Variants: Oghma, Ogmios, Grianainech (sun face), Cermait (honey-mouthed).

Pwyll - Wales; God of cunning, virture. Called Pwyll pen Annwn (Pwyll, head of Annwn) because he replaced Gwynn ap Nudd as ruler of the underworld at one time.

Taliesin - Wales; God of magic, music, poetry, wisdom, writing. Known as Prince of Song, Chief of the Bards of the West, and Patron of Druids, he was a great magician, bard, and shapeshifter who gained his knowledge from the goddess Cerridwen directly.