Irish Classification Statements Using the Pattern IS + ADJECTIVE + OBJECT CLASS + SUBJECT / PRONOUN.
I am looking at sentences of the following sort:

Is deas an fear é.
= He is a nice man.

If the subject is plural though, does the adjective have to match in number in this classification statement? For instance, should I write...

1. Is deas na fir iad. = They are nice men.

-- or --

2. Is deasa na fir iad. = They are nice men.

Question

Irish Classification Statements Using the Pattern IS + ADJECTIVE + OBJECT CLASS + SUBJECT / PRONOUN.
I am looking at sentences of the following sort:

Is deas an fear é.
= He is a nice man.

If the subject is plural though, does the adjective have to match in number in this classification statement? For instance, should I write...

1. Is deas na fir iad. = They are nice men.

-- or --

2. Is deasa na fir iad. = They are nice men.

Answer

No, the adjective doesn't change: "Is deas na fir iad" is the correct form.
The adjective only changes to agree with the noun when it comes immediately after it (attributive adjectives), otherwise gender and number don't affect it (in this respect Irish works differently to the Romance languages (Spanish, Italian, French, etc.), in which even predicative adjectives have to agree).