I use the Teach Me! Irish learning software, which seems pretty good, but one major problem it has is that it is sometimes vague and lacking in explanation/consistency. So, I was just wondering what the difference between "Is gá" and "Tá gá" is?

Is gá = It is necessary
Tá gá le = There is a need for

Is gá airgead a chur i dtaisce chun teach a cheannach.
Tá gá le hairgead chun teach a cheannach

So, is there certain context each is required for, or could you use either anywhere?

Scooby: Please help me understand this use of the copula. I always look for two nouns and/or
pronouns-nouns with the copula. I don't see that in the sentence. Could "gá" mean necessity? It's just not a word I am too familiar with. Would appreciate your help.

Yes, gá can be referred to as necessity.

Gá is a noun. It translates as "necessity/need/requirement."

It may help to think of this sentence as being similar to "is maith le." While we translate this idiomatically as "I like," it's literally "is benefit/goodness with." Irish resorts to noun constructions in a lot of places where English would use verbs.

Redwolf

Alright...So do they change too much of the meaning of the sentence if you change which one you use?

This is just a very general observation and there are probably exceptions
Is gá is used when an action is needed (or not, as the case maybe) - Is gá é a dhéanamh - It has to be done; ní gá a rá go... - It doesn't have to be said that....; is gá dom fanacht - I have to wait.
Tá gá seems to be used with a mateirial object - Tá gá aige leis an carr - He needs the car.
There is probably a lot more to it.

I'll keep that in mind