There's just a few rules of grammar that have been annoying me recently. I know the difference between lagiolraí and tréaniolraí, but are there any specific rules for forming the nominative plural? For example, "marcach" and "bealach" are both in the 1st declension, but the plurals are "marcaigh" and "bealaí" respectively. Are there any rules or guidelines that dictate this difference?

Also, if a phrase in the Genitive comes within another genitive construction, is every noun just placed in the genitive? For example, "the man's coat" is "cóta an fhir", but would "the colour of the man's coat" be "dath an chóta an fhir", with both cóta and fear being in the tuiseal ginideach?

As well, I know about the Tuiseal Tabharthach, but is it still widely in use, or is it a more archaic form than standard use?

Go raibh maith agaibh!
Cathal

For your first question, there is no rule and no way to predict the irregular plural of "bealach". The regular plural pattern for the 1st declension would seem to dictate that the plural of "bealach" should be "bealaigh", but it isn't. It's "bealaí", which is an exception and you need to memorize it individually.

Your second question, if you have a phrase containing a noun plus an article plus another noun in the genitive, such as "cóta an fhir", and you want to put this whole phrase in the genitive, then the way to do it is as follows: (1) leave the first noun in the nominative, (2) aspirate the first noun if possible, and (3) do NOT place an article before the first noun. Result: "dath chóta an fhir", the colour of the man's coat. More information here: http://www.cainteoir.com/Gramadach.aspx?Abhar=ainmfhocal_alt_ainmfhocal

The answer to your question about the dative case is quite simple, the dative case is not widely used and yes, it may sound a little archaic to some speakers. Standard Irish allows it but does not require it.

Go raibh míle maith a't, is mór an chabhair é sin!!

Re tuiseal tabharthach - it exists and is widely used, just its form is very often the same as tuiseal ainmneach. When you see things like "an t-im", but "ar an im" you should realize that "im" in the first case and "im" in the second case although unaltered in form are not the same, as mutation is different. Thus, even if there were no forms of dative different from nominative - you would still say that there dative in Irish.

Yes, that's another and equally valid analysis of the same phenomenon.

'Sea, bhí's a'm gurb ionann an Tuiseal Ainmneach agus Tabharthach go minic, ní raibh ann nach raibh a fhios a'm faoi úsáid na hUimhreach Iolra!!