Can you go to jail if you forge a prescription in UK/Ireland for Xanax?

The fact Xanax is an American brand name so that doesn't exist in the UK/Ireland. The drug is not available on the NHS but the class C drug of a branded EU name can be obtained on a prescription from a private doctor, same in Ireland, not sure where you think you could get a prescription form from to be able to forge it

Yes - forgery is illegal. 

In theory, yes.  The chances are that if you required it for your own use, you'd be fined.  If you were a dealer, you'd be jailed.

We don't have Xanax by that name in the U.K.
We also call jail Prison, so you don't live here do you? So the point of your Q is useless.

You may not get away with that.  It is not commonly prescribed in the British Isles.