"She has green eyes and she has blond, curly hair."
Translation:Tá súile glasa aici agus tá gruaig fhionn, chatach uirthi.
The noun following the number 2 is always in the singular, but there is no number expressed here. (The noun following the cardinal number 3-10 is also usually in the singular).
Tá súil glasa aici would just mean "she has a green eye".
Your reference to "the dual case" suggests that this footnote on GnaG might be helpful:
The dual stood earlier in Indoeuropean languages next to the singular and the plural
The remnants in Irish (as a composite of singular/plural) are:
1.the singular article (an) preceding two e.g.: an dá bhád = the two boats (instead of the otherwise necessary plural article na preceding 3-10: na trí bhád = the three boats)
2.if it exists, a noun after two is in the dual form (= dative singular form) e.g. dhá bhróig =two shoes
3.otherwise, the nominative singular form of the noun always comes after two e.g.: dhá bhád = two boats (after 3-10 also the plural is possible)
4.following adjectives are then still in the plural e.g. an dá bhád mhóra = the two big boats (like with 3-10)