"Cosnaíonn siad céad euro."
Translation:They cost a hundred euro.
According to this article, "In official documents, the name "euro" must be used for the nominative singular in all languages, though different alphabets are taken into account and plural forms and declensions are accepted. In documents other than EU legal texts, including national legislation, other spellings are accepted according to the various grammatical rules of the respective language."
Also: "In Irish, the words euro and cent are used without change in spelling or pronunciation, and immune to the regular rules of Irish mutation after numbers. The masculine noun eoró (plural eorónna) has been coined from the word Eoraip ('Europe'), and ceint (plural ceinteanna) has been in the lexicon since at least 1959. The words eoró and ceint are attested in printed literature, but are very rarely encountered."
And yet an exception was made for Bulgarian in official documents — its nativized евро (“evro”) winning out over the strict transliteration еуро (“euro”). Presumably its Cyrillic alphabet was the deciding factor, since the linguistic differences in languages like Irish, Slovenian, and Latvian weren’t sufficient to sway the EU Council to make exceptions for their Latin-alphabet nativized forms.
She says cosnaíonn sibh not cosnaíonn siad. At least, that's what I hear.