The Celtic Pantheon is one of the few open belief systems for pagans to adopt. This is a basic list of those deities and is in no way all inclusive or complete. These are just from what I could find from multiple sources on the internet and a few books that I own. Their associations are very basic, and if you see a deity you are interested in, you should definitely do your own further research. Also, please be aware that Celtic is a blanket term and that some of these deities DO translate across the different sects of Celtic cultures, but some of them are specific to a distinct region. For example, Irish Celtic, Scottish Celtic, etc. Again, its very important to do your own research. This is just meant to be a very basic starting point.
Abellio: God of the Apple Tree
Aine: The Fairy Queen. Goddess of Summer, Love and Fertility.
Airmid: Goddess of Medicinal Herbs and Spring.
Amaethon: God of Agriculture and Luck
Andraste: Goddess of the Moon, Rabbits and Magic
Angus Og: God of Youth and Beauty
Anu: Goddess of Plenty
Arawn: God of the Underworld, Terror, Revenge and War
Artio: Goddess of Wildlife
Babd Catha: Goddess of War
Bel: God of Purification, Fire and the Sun
Bran: God of Health
Branwen: Goddess of Love and Beauty
Brighid: Goddess of Fire, Water, Hearth, Protector of Women
Bris: God of Fertility and Agriculture
Caer Ibormeith: Goddess of Sleep and Dreams
Cailleach: Goddess of Plague and Disease
Camalus: God of War and the Sky
Cernunnos: The Green Man. God of Life, Fertility, Animals and the Underworld
Cerridwen: Goddess of the Moon, Grains and Nature
Creidhne: God of Metalworking
Creiddylad: Goddess of Flowers and Love
Cyhiraeth: Goddess of Streams. Her scream foretells death.
Dagda: The Great King. Father of All. God of the Earth.
Danu: Mother of the Dagda. Goddess of Creation and Water.
Diancecht: God of Healing and Medicine
Druantia: Goddess of Trees, Creativity and Passion
Epona: Goddess of Horses, Prosperity, Springs and Mountains
Eostre: Goddess of Springtime
Flidais: Goddess of Forests, Woodlands and Wild Things
Goibhniu: God of Blacksmiths, Weapons and Brewers
Gwydion: God of Magic, Illusion and Enchantment
Latis: Goddess of Lakes, Love and Alcohol
Lir: God of Waters and the Sea
Macha: Goddess of War, Life and Death
Maeve: Goddess of Earth, Fertility and War
Morrigan: The Great Queen. Goddess of War, Death and Fertility.
Neit: God of War, Blood, and Sacrifice in Battle
Nuada: God of Musicians and Artists
Olwen: Goddess of Rebirth, Springtime and Flowers
Rhiannon: Queen of the Sirens. Goddess of Horses, Birds and the Moon.
Rosmerta: Goddess of Wealth
Tahesin: God of Song
Taranis: God of Thunder
(A Family Tree for the Celtic Pantheon, Credit to Akherontis of GaiaOnline)