In grammar what does 'Abairtín' refer to? I know 'abairt' is a sentence and that 'ín' is a diminutive. If that makes it a 'little sentence' what does that mean?
DMD

It might mean clause - have you got an example?

Gearrchúrsa Gramadaí - Brian Mac Giolla Phádraig
lth. 99
An Chopail: Tagann an Fhaisnéis (ainmfhocal, forainm, nó abairtín) díreach tar éis na Copaile.
Some where in this very same book I think clause might be Chlás and phrase phrás, though I can't find it right now.

The Copula: The information - noun, pronoun or small sentence (fragment?) comes directly after the copula.

I think "sentence fragment" is the best translation.

Have a look at these google results for more examples:

http://www.google.ie/search?sourceid=navclient&hl=en-GB&ie=UTF-8&rls=GGLJ,GGLJ:2 006-39,GGLJ:en-GB&q=abairt%c3%adn

Irish Nouns (Andrew Carnie) gives it as "phrase".

According to both Ó Donaill and de Bhaldraithe abairt can mean either sentence or phrase; and de Bhaldraithe specifies Frása for the grammatical term.

But here I agree that it most likely means phrase, although I think it could refer to something which wouldn't work "stand alone".

I think it means "expression." Not expression, as in expressing one's opinion or thoughts, but as in a small thought, or afterthought - something small, but not necessarily trivial. A (small) expressive segment of speech.