Just a small point: while the dual number is indeed a rare phenomenon, Irish is not the only European language to retain it. Slovenian also have the dual number.

Cén fáth nach deirtear “Chríochnaigh mé sé *mbliana* staidéir…” ?

Freagra: The numbers from seven upwards are NOUNs. Just as we could say “a score OF mockers” or a dozen OF eggs” we say in Irish “a seven of years” ie “seacht mbliana”. But from 3-6 we have a regular plural: sé bliana. In the standard dialect this use is confined to the word ‘bliain’, which is used very frequently with numbers. In the trad. Munster dialect it is retained with any noun.
Irish is the only modern European language that retains the DUAL number: dhá chapall but trí (etc) capaill. The dual is identical in form with the dative. In the Standard Grammar only a small handful of words keep the dual: mo dhá láimh: also ag an láimh [dative]. But in the standard grammar, the dual form has invaded the numbers up to six: trí chapall where the older (and Munster) was trí capaill.

The dative shows up only in the Second declension. It is ‘optional’ in the standard grammar, but I don’t seeanyone still using it: fuinneog, a window, but ag an bhfuinneoig, at the window. Also, dhá fhuinneoig, two windows.