I will edit this and add an explanation for lenition here when I figure it out. Something about the definite article feminine word starting with "f".
That’s the explanation – feminine definite article causes lenition.
And since the f disappears when lenited, the article takes the form an before a vowel (instead of regular a’ before a lenited consonant, as in a’ phàirc ‘the park’).
thanks, guys, very helpful!
Just a few questions ago, the very same sentence used "fear eile" rather than "te' eile." Which is correct?
That depends on what it refers to. Here tè is correct because it refers to forca which is a feminine noun. fear refers to nouns with masculine gender.