We don't have synonyms for Celtic Sea yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are baltic sea.
(n) :
A sea in British Isles, Europe in the Atlantic The area of the Atlantic Ocean off the south coast of Ireland bounded by Saint George's Channel, the Bristol Channel, the English Channel, and the Bay of Biscay.
(n) :
A sea in British Isles, Europe in the Atlantic The area of the Atlantic Ocean off the south coast of Ireland bounded by Saint George's Channel, the Bristol Channel, the English Channel, and the Bay of Biscay.