Táim ag déanamh stáidéir ar an tuiseal ginideach (go bhfóire dia orm).

In the 1st declension, when forming an tuiseal ainmneach uimhir iolra, nouns ending in -ach or -adh change to -(a)í
(éadach > éadaí srl.)
But sometimes -ach changes to -(a)igh, mar shampla: taoiseach > taoisigh (making it a weak instead of a strong plural).

Are you just supposed to know intuitively which ending it changes to?

Go raibh maith agaibh roimh ré as aon cabhair.

I would say that the general rule is that where the singular ends in -(e)ach, then the nominative plural, and genitive singular, is -(a)igh in the First Declension...so that the genitive plural looks like the nominative singular and the genitive singular looks like the nominative plural. There are always exceptions.

First declension nouns (m1) are all masculine. First declension -ach changes to -aigh in the genitive singular and nominative plural, then back to -ach in the genitive plural, as Scooby has explained.

Second declension nouns (f2) are all feminine. Second declension -ach changes to -aí in the genitive singular and -acha in the nominative plural, then back to -ach in the genitive plural.

Those apply to polysyllabic words, where the -ach ending is a syllable in its own right, i.e., taoiseach, éadach, but not to monosyllabic words like teach, or beach.

The plural change may be overridden by a stronger plural ending, i.e., -anna, etc., in which case the nominative plural and the genitive plural both have that strong ending.

Éadach is an exception, being masculine, but having -í in the plural. You just have to think of the -í ending as overriding the general rule in such cases.

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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).

When you have it all figured out, summarise it all and teach those of us who have great intentions, but..... Go n'éirí leat!

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A Mhaighdean...

Every source I try to learn the tuiseal ginideach from does it so differently. What I took from this source:
http://www.nualeargais.ie/gnag/1dekl.htm

was that for the nominative plural -ach changes to -aí, but sometimes it changes to the weaker -aigh

'rarer, strong plural

of words ending in -ach or -adh with suffix -aí
(with -ach also weak plural in -aigh'

but I'm after realising I probably took that up the wrong way and most of the time you do the same as the genitive singular, and change it to -aigh.

And I didn't make myself very clear to start off, what I meant was how are you supposed to know the exceptions - you just learn them? There's just such an incomprehensible number of exceptions to everything in Irish, you don't get any nice list to learn off!


Exam stress making me resent whatever jokers made up Irish grammar... Thanks for your help Breandán agus Scooby.

Haha déanfaidh mé cinnte nuair a bhfuil tuiscint maith a'm air! But better teachers than I have failed there already. (:

Another thing that's contributing to my foul mood, Ó'Dónaill doesn't give the declension of nouns, only their gender? How are you supposed to figure out what declension nouns fit into?

I prefer to just take each word individually. When I learn a new word, I practice saying it in different cases, singular and plural, with and without articles and prepositions.

Definitely not a foolproof method, but I just find it takes me too long to work out the declensions in the middle of a conversation. By the time I get the word out, I've forgotten what I was going to say.

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Táim ag foghlaim fós. Fáilte roimh gach aon cheartúchán.

Try English! It starts off nice and simply: add an 's' to make a plural.
Then you think about 'sheep' and you realise that the singular and plural are the same. And then you encounter words like man/men (but pan/pans); mouse/mice (but house/houses); child/children; radius/radii; referendum/referenda; ox/oxen. And that is without much thought - seriously not easy if you are learning from scratch. Actually, it would be interesting to hear from someone who learned both English and Irish as additional languages as to which they think is more straight forward.
Not that my ramble is of any help to you at all. Sorry!

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Wiktionary has a good deal of Irish vocabulary. Just type a noun into the search box in the top right corner, and they should give you the different forms.

http://en.wiktionary.org/wiki/Wiktionary:Main_Page

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Táim ag foghlaim fós. Fáilte roimh gach aon cheartúchán.

someone like me?
I think most plurals are more regular in English. In English there are only a few exceptions that I learnt by heart when I was at school, and there are some other exceptions I learnt later (like fish, deer etc, I never learnt they were both singular and plural, actually I learnt that in an Irish class ).

In Irish it's more complicated but in several cases, several plurals are possible (carr> cairr, carrannaì etc) so with some words, if you don't use the right ending, people will understand anyway and won't even be surprised, maybe

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