Aes Sedai: In Celtic myth, there are beings known as the Sidhe (pronounced 'shee'), which literally means "people of the (fairy) hills". It is the Gaelic name for the fairies in both Ireland and the Highlands of Scotland. Specifically, the Aes Sidhe were 'the people of the hills', collective name for the old Irish gods who dwell in hills. According to the Encyclopedia Mythica, they "still live as invisible beings… In a just battle, they will fight beside mortals. When they fight, they go armed with lances of blue flame and shields of pure white". [Scott Dwiggins]
Bel Tine: Beltane, a Druidic holiday
Birgitte: Brigit/Brigid, Celtic goddess of fire, poetry, smithery, and/or healing.
Portal Stones: Evocative of the famous cairns and "standing stones" that dot the British Isles. Dustin Clayton notes, "The cairns were believed to be plexuses [in-between places] of a sort, except that they were plexuses to another world, better known to some as the Fairy World or Otherworld. The legends of cairns are that if you were to walk around a cairn three times in a 'sunwise circle' you will find an entrance to the cairn, especilly when it was sunrise or sunset. The cairns themselves weren't passageways to other worlds, but were meant to be warning signs to the unwary that strange powers rested at these places, and those who didn't understand them might be caught in them."
"Rhiannon at the Tower": Rhiannon from Welsh mythology. The closest connection between her and a tower that I could find is the following from the story of Manawyddan: Manawyddan, his wife Rhiannon, her son Pryderi, and his wife Kicva were out hunting. They came upon a mysterious castle. Their hounds ran into the castle, and didn't come out. Pryderi went in after them, and saw a large gold bowl. He touched the bowl, and became frozen. After a while, Rhiannon went in to look for him, got similarly stuck, and then the castle disappeared. (They finally got released.)
Tam Lin: [L:NS] notes that Tam's full name is "Tamlin al'Thor". Tam Lin was an Irish knight stolen away by the elf queen and forced to guard her kingdom, until the love of a mortal woman brought him back to our world. [Alex Bertran]
Tel'aran'rhiod: In Celtic myth the Goddess has three aspects, the Maiden, the Mother, and the Crone. The names of the aspects change from myth to myth, but in some versions the Mother aspect is called Arianrhod. "Arian" means "silver", while "rhod" is "wheel" or "circle". Arianrhod appears in the Mabinogian as the sister of Gwydion (or his wife, or both), and is associated with the moon, fertility, the stars, the aurora borealis, and time. [Kathy Putnam, Sonja Pieper]
Tuatha'an: Tuatha De Danann, "People of the Goddess Danu." A magical race of beings who were supposed to have inhabited Ireland at the time of the arrival of the Milesians (i.e. humans).
Wild Hunt: Wild Hunt, of Celtic legend. The Wild Hunt is a supernatural force that sweeps across the land at night. The actual object of the Hunt varies from place to place. In some areas it searches for anything that might be unfortunate enough to be in its path. Others say it hunts evildoers. The leader of the Hunt also varies. In Celtic Britain it is usually led by Cernunnos, the horned god. In Wales it is led by Gwyn ap Nudd, and sometimes Bran. After the Anglo-Saxons had settled in England, Cernunnos became Herne the Hunter. The Wild Hunt also appears in Teutonic myth, its leader being Woden or Odin. [Encyclopedia Mythica]