Nouns in Early Modern Irish are divided into two genders: masculine and feminine.
Nouns ending in a broad consonant (a, o, u) are typically masculine, while nouns ending in a slender consonant (e, i) are typically feminine.
The old neuter gender is generally no longer recognized in Early Modern Irish. Neuter
-o, -io and -s stems are typically treated as masculine.
In the transition from Old Irish to Early Modern Irish, a significant number of nouns have changed gender and thus are found in both masculine and feminine forms. [The Bardic Poems of Tadhg Dall Ó hUiginn 1, p. lxvii]
Some nouns which show traces of their old neuter gender: arm, bé, buaidh, céad, dál, diombuaidh, fíor, lá, loch, magh, rath, síol, sliabh, teach, tráth, trian. (Bergin, Stories From Keating, p. xi)
Often eclipsis spreads beyond its original limits, for example the nominative síol nEóghain, where síol was formerly neuter, gives rise to the genitive síl nEóghain. (Bergin, Stories From Keating, p. xi)