tá mo gruaige fliuch = My hair is wet

tá mo chuid gruaige fliuch = My hair is wet?

cuid = some , part of , portion.

whats happening here? Ive also seen this :

Mo chuid Gaeilge = My Irish

Would not , Mo Ghaeilge say the same thing?

Whenever there's an indefinite quantity of something in question, one must use "cuid". If you knew all there was to know about Gaeilge and you owned it exclusively, you could say "mo Ghaeilge", but as this is something that is SHARED by a large number of people with variation in quantity and quality, you have to say "mo chuid Gaeilge". The gruaig example is a bit puzzling. Sure, hair is an indefinite quantity, but so is skin, and I've seen "mo chraiceann". I've also seen "mo chuid fiacla" (my teeth). Perhaps it's because, like the English word "hair", gruaig is both singular and plural. So "chuid" is used to make the distinction.

Tá mo ghruaig fliuch
or
Tá mo chuid gruaige fliuch

are equivalent. Its a language. There's more than one way to say it!

Celtoid's advice is good.

I've always looked at "cuid" as "my portion" of that indefinite quantity. As Celtoid stated, if it's yours and yours exclusively, or if it's something that you have full and total control/command over, then I guess you could leave it off. But, whether you put it in or leave it out, you will be understood.

Le meas,
An mac Díobhlásach

I suppose if I tried to explain the usage of "cuid", I'd go along the lines of...

you use it with a fundamental component part. For example, for a car, the fundmental necessities would be:

wheels,
engine,
transmission,
brakes

So I'd you "cuid" with them, if you get my drift. So if you look at a person:

hair
wealth
etc.

you'd use cuid.


Also: I've seen it used in the form of "of his". For example, that's a bike of his:

Is rothar de chuid Sheáin é.