"The restaurant's menu."
Translation:Biachlár na bialainne.
It's not a plural article. In the genitive singular, feminine nouns use na instead of an.
I have read that nouns are very likely to be feminine when they end in a slender consonant, end in -eog, -óg, -lann, are multi-syllable nouns ending in -acht or -íocht, are names of countries and languages, are abstract nouns ending in -e or -i