This is by no means a complete list of Celtic Goddesses, 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.

Aine - (AN-yuh) Ireland; a woman of the Leanan Sidhe (Sweetheart of the Sidhe). Some said she was the daughter of Manannan, some said she was the Morrigan herself. There was a stone, Cathair Aine, belonging to her and if anyone sat on the stone, they would be in danger of losing their wits, sit three times and they would lose them forever. Aine was very revengeful, and it was not a safe thing to offend her.
Aine of Knockaine - (AN-yuh of knock-AN-yuh) Ireland; moon goddess and patroness of crops and cattle; associated with the Summer Solstice. Also Aine Cliach, and Cnoc Aine.
Anu - Ireland; goddess of plenty and Mother Earth. Greatest of all Irish goddesses, deity of cattle, health, fertility, prosperity, and comfort.
Aoibhell - (Evill) Ireland; another woman of the Sidhe, she made her dwelling in Craig Liath. Legend has it that she gave a golden harp to Meardha, Murchadh's son, when he was getting his schooling at the Sidhe in Connacht and learned of his father's death. Whoever heard the playing of the harp would not live long afterward. It was this harp that Cuchulain heard the time his enemies were gathering against him at Muirthemne, and he knew by the sound that his life was near its end.
Arianrhod - Wales; goddess of beauty, fertility, and reincarnation. Known as Silver Wheel and the High Fruitful Mother, the palace of this sky goddess was Caer Arianrhold (Aurora Borealis). Keeper of the Silver Wheel of Stars, a symbol of time and karma. Her ship, Oar Wheel, carried dead warriors to Emania (Moon-land).
Badb - (Bibe) Ireland; goddess of enlightenment, inspiration, life, wisdom. Sister of Macha, the Morrigan, and Anu, the name of this goddess means "boiling," "battle raven," and "scald-crow". Known as Cath Bodva in Gaul. A Mother Goddess and Triple Goddess, Badb's cauldron boiled with the ever-producing mixture that produced all life. Variants: Badhbh, Badb Catha.
Banba - Ireland; one of a triad of goddesses that included Fotia and Eriu.
Blodeuwedd - Wales; goddess of flowers, lunar mysteries, wisdom. Known as the Ninefold Goddess of the Western Isles of Paradise and Flower-Face, goddess was created by Math and Gwydion as a wife for the god Lleu. Her symbols were the owl and the moon. Variants: Blodwin, Blancheflor.
Boann - Ireland; goddess of the River Byone and mother of Angus Mac Og by the Dagda. She held the powers of healing. Variants: Boannan, Boyne.
Branwen - Wales; goddess known as Venus of the Northern Seas was the deity of love and beauty. Daughter of Llyr and one of the three matriarchs of Britain.
Brigit - Ireland; goddess of agriculture, fire, healing, inspiration, learning divination, occult knowledge, poetry, prophecy, smithcraft. Her Gaelic name of Breo-saighead means "fiery arrow" or "fiery power". Celts often referred to her as being three in one - the Triple Brigits or the Three Mothers. An ever-burning fire was kept in her honor by her nineteen priestesses who lived in a sacred temple at Kildare. She was also a daughter of the Dagda. Variants: Brid, Brig, Brigid, Brighid.
Caer Ibormeith - Ireland; goddess of sleep and dreams; and perhaps a less violent version of Mare; daughter of Ethal Anubail, a faery king of Connacht. She often took the form of a swan who lived on a lake called Dragon's Mouth, and wore a copious golden chain with 130 golden balls on a silver chain about her slender neck. She was loved by Aengus MacOg, god of young love. When he awakened from a dream of her he sought her out. After he found her, he too became a swan, and the two of them flew and sang the sweetest, most restful music ever heard upon this earth. Together they flew away to Bruigh na Boinne, his megalithic site north of Tara, where they sang so wonderfully that the whole of Ireland fell into a peaceful sleep for three days and three nights.
Caillech - Ireland, Scotland; goddess of disease and plague. A Destroyer, or Crone, goddess, she was also called "Veiled One". As the Crone, she ruled with the Maiden and the Mother. Dogs guarded the gates of her afterworld realm where she received the dead. Celtic myth has her gatekeeper dog named Dormarth "Death's Door". Irish bards who could curse with satire were often called cainte "dog".
Cerridwen - Welsh; goddess of death, initiation, inspiration, magic, regeneration. Known as a moon goddess, Great Mother, and grain deity; wife of the giant Tegrid. She brewed a magical potion of wisdom in her cauldron, and forced the young Taliesin to stir it for a year and a day. When he accidentally swallowed the last three drops, he was transformed into a bard. Welsh bards once called themselves Cerddorion "sons of Cerridwen," meaning they received their initiation from Cerridwen herself. Variants: Caridwen, Ceridwen.
Creiddylad - Wales; goddess of flowers, love. A daughter of the sea god Lir, connected with the festival of Beltaine and called the May Queen. Variants: Creudylad, Cordelia.
Danu - Ireland; Mother of the Gods, she was goddess of rivers and wells, magic, plenty, wisdom. Possible aspect of Anu; ancestress of the Tuatha De Danann. Variant: Dana.
Don - Ireland, Wales; in Ireland, goddess who ruled over the Land of the Dead. In Wales, goddess of sea and air. For both, generally a goddess of the elements, communicating with the dead.
Donn - Wales; the sea goddess.
Druantia - several Celtic areas; goddess known as Queen of the Druids and Mother of the tree calendar.
Eadon - Ireland; nurse of poets
Eiru - Ireland; daughter of the Dagda, her alternate name, Erin, was given to Ireland.
Elaine - Wales, Britain; a Maiden aspect of the Goddess, she was later transformed in the Arthurian sagas.
Epona - Britain, continental Gaul; goddess of horsebreeding, healing spring, prosperity. Called Divine Horse and the Great Mare, the goddess of horses was acknowledged and worshipped by Roman soldiers. Her symbols were horses and dogs.
Flidais - Ireland; goddess of forests, wild creatures. A shapeshifting goddess who rode in a deer-drawn chariot.
Macha - Ireland; goddess of cunning, death, sheer physical force, war; protectoress in both battle and peace. Known as Crow, Queen of Phantoms, and the Mother of Life and Death, she was honored at Lunasa. Variants: Mania, Mana, Mene, Minne.
Margawse - Wales, Britain; originally a Mother Goddess, she was transformed in the later Arthurian sagas.
Morrigan - Ireland, Wales, Britain; a shapeshifting war goddess of lust, magic, prophecy, revenge, war. Known as Great Queen, Supreme War Goddess, Queen of Phantoms, and Specter Queen, she kept company with Fea (hateful), Badb (fury), and Macha (battle). Variants: Morrigu, Morrighan, Morgan.
Niamh - (Nee-av) Ireland; possible form of Badb, this goddess was called Beauty and Brightness and helped heroes at death.
Scathach - (Scau-ahch) Ireland, Scotland; goddess of healing, magic, martial arts, prophecy. Called the Shadowy One, She Who Strikes Fear, and the Dark Goddess, she was a warrior woman and prophetess who lived in Albion, possibly on the Isle of Skye, and taught martial arts. Variants: Scota, Scatha, Scath.
White Lady - all Celtic countries; goddess of death and destruction. Called the Dryad of Death and Queen of the Dead, this goddess was a Crone aspect of the Goddess.