"Na h-uinneagan beaga."
Translation:The small windows.
It is not the noun that determines if the "a" is added or not. It's the adjective itself. There is not a lot on adjective declension that I have been able to find. In general, if the adjective is monosyllabic and you have a plural noun, then you add the "a" (or an "e" if the preceeding vowel is small in the adjective)
If the adjective has more than one syllable, it generally ends like the nominative singular.
There is a free ebook called The Principles of Gaelic Grammer by John Forbes that has a helpful section.