Almost right. 1a applies to nouns beginning with a vowel or "f", but only in the "f" is followed by a vowel (e.g. "d'fhear", but "do fhrancach" = "to a rat"). I think a simpler way to look at it is that "do" lenites a following consonant, then if the resulting noun begins with a vowel sound (as you know, "fh" is silent, but "fhr" and "fhl" sound like "r" and "l") the "do" is reduced to "d'" to avoid two vowel sounds coming together. "Don" doesn't lenite "d" or "t", in the same way as the article by itself never lenites these consonants (don duine, don teach); in some dialects (especially in the north) it prefixes "t" to "s" (don tsagart), as the article regularly does with "s" in situations in which it would otherwise lenite. I think in some dialects this only happens if the noun is feminine though. In some dialects in the south-west, there is eclipsis rather than lenition after "don" (I think whether "d" and "t" are eclipsed varies according to sub-dialect). Basically you just need to remember (for Ulster Irish) that preposition + article causes lenition, and the lenition follows the rules for lenition after the article (e.g. nominative singular feminine, genitive singular masculine). In Munster preposition + article tends to eclipse and Connacht is somewhere in between. "Don", "den" and "sa" often lenite even in many dialects that prefer eclipsis.