I'm trying to translate this, but as the whole idea of dividing the year up into twelve months must have seemed quite contrary to the traditional Irish concept of the year, perhaps there exist other rhymes or poems that describe the way seasons and months are presented.

Thirty days has September,

Tá tríocha lá Méan Fómhair,

April, June and November.

Aibreán, Meitheamh is mí na Samhna orthu.

All the rest have thirty one,

Tá gach eile lá is tríocha orthu,

Except February alone,

Ach Feabhra aonair, (or amháin ?)

Which has twenty eight days clear,

atá fiche a hocht lá iomlán,

And twenty nine days in each leap-year.

Agus fiche a naoi lá gach uile bhliain bhisigh air.

Corrections welcomed

Thirty days has September,

Tá tríocha lá ag Meán Fómhair,



Déarfainn: tá (aon) lá is tríocha ag an fhuílleach
(nó: tá aon lá déag is fiche...)



ach amháin Feabhra



a bhfuil ocht lá is fiche aige



agus naoi lá is fiche aige gach uile bhliain bhisigh

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Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne

Irish calendar concepts had gone during christianization.

As Lughaidh pointed out, it is:

Tá 30 lá ag Meán Fómhair.

Doing so, you have to put ag before every single month: ag Meán Fómhair, ag Aibreán, ag Meitheamh ...
This makes the sentence quite long.

So, it's a good idea to use a pronoun like orthu or rather acu instead.
But in this case all month names must be put outside of the sentence:

Meán Fómhair, Aibreán, Meitheamh. ..., tá 30 lá acu. (Sept., April, June, ..., they have 30 days)

Or you use a phrase like this:

Tá 30 lá ag na míonna seo: Meán Fómhair, Aibreán, Meitheamh ... (These months have 30 days: ...)

Go raibh maith agaibh

I think it should be acusan since you are contrasting them with the previous set.

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Múinteoir Gaeilge - Irish Teacher
My "specialty" is Connemara Irish, particularly Cois Fhairrge dialect, but I can also speak Ulster and Munster Irish with native-level pronunciation.
Is fearr Gaeilge ḃriste ná Béarla cliste, cinnte, aċ i ḃfad níos fearr aríst í Gaeilge ḃinn ḃeo na nGaeltaċtaí.
Gaeilge Chonnacht (GC), go háraid Gaeilge Chois Fhairrge (GCF), Gaeilic Uladh (GU), Gaelainn na Mumhan (GM), agus Gaeilge an Chaighdeáin Oifigiúil (CO).

In 'Teach Yourself Irish' by Myles Dillon and Donncha Ó Cróinín, they have a different way of saying this - ie:

Tá deich lá fichead/is fiche san Aibreán agus aon lá déag ar fhichid i mí na Bealtaine. and -

Níl ach deich lá fichead/ar fhichid san Aibreán, sa Mheitheamh agus i mí Mheán an Fhómhair.

so does this work ?

Níl ach deich lá ar fhichid i mí Mheán an Fhómhair,
San Aibreán, sa Mheitheamh agus i mí na Samhna.
Tá aon lá déag ar fhichid san fhuilleach/chuid eile
Ach amháin mí na Feabhra
A bhfuil ocht lá is fiche inti
Agus naoi lá is fiche gach uile bhliain bhisigh inti.

I have just been sent this version by Peadar Ua Laoghaire by David Webb, for which I'm very grateful -

Tríochaid lá 'bheirid leó
Samhain, Abrán, na Mithimh teó.
Lá sa mbreis ag gach mí eile
Ach an Feabhra thiar ar deireadh,
Gan aige ach ocht lá fhichead -
Naoi lá fhichead, i mbliain bhisigh.

Not totally mícheart, but ar fhichid usually with ... déag
as in:



With 2-10 genitive of fiche, fichead is used: deich lá fichead.

Ceart go leor - but the way this is presented in the book, you'd think either would be permissible. Go raibh maith agat.