Part II - Etymology
§8. — The Article
The article an, the, is inflected thus : Singular. Nom. an ; gen. masc. an, gen. fem, na ; Plural, na. The dative is formed by an, with a preposition. After a preposition ending in a vowel the a of the article is dropped, as do'n aṫair, incorrectly written don aṫair, to the father.
The form naiḃ (Zeuss, p. 238,) was anciently used in the dat. plural, but is not to be met with in modern Irish.
Certain prepositions when followed by the article, assume an s, e.g. le with, llesan, with the; tré through, trésan, through the; iar, after, iarsan, after the. This fact we conceive to be explicable only by assuming that the s belongs in reality to the article, and not to the preposition.
The article causes the following changes in the initials of nouns.
1. If the noun begins with a vowel, the article prefixes t to the nominative singular of masculines, and h to the genitive singular of feminines : it prefixes h to all the cases of the plural except the genitive, to which n is prefixed. See below, No. 2.
2. If the noun begins with a mutable consonant, except d, t, s the article aspirates the initial mntables of masculines in the genitive, and of feminines in the nominative.
After do and de the article aspirates in the dative of both genders. In some parts of Ireland eclipsis is used instead of aspiration. Ann, in, also aspirates with the article.
In the genitive plural all initial mutables, including d and t, are eclipsed, except s. . The reason of this is, that the genitive plural originally ended in n, as innan, nan. This n appears before vowels as mentioned in No. 1, and so in annsan tosaċ, in the beginning.
3. If the noun begins with s, followed by a vowel, or by l, n, or r, wherever the article would aspirate o ther consonants, it, in this case, eclipses s by prefixing t. The origin of this t, and also of that mentioned under No. 1, has not yet been satisfactorily explained.
4. If the noun should begin with l, n, or r, or s before a
mute, the article causes no change whatever, or in the singular
of those beginning with d or t.