Hi there,
I'm a 28-year-old guy from Yugoslavia trying to learn some basics of Irish Gaelic. I wonder how to say "Yugoslavia" or "Serbia" in Irish, and how to say "Yugoslav" or "Serbian".
I appreciate someone's quick help,

Go raibh míle maith agaibh,
Slán

Sasa, a chara,

Yugoslavia = an Iúguslaiv
Yugoslav = Iúgaslavach or Iúgaslavach (noun or adjective)
Serbia = an tSerbia
Serb = Serbiach

Le meas,
Larry.

... and Serbian = Serbis (language) or Serbiach (nationality)

Le meas,
Larry.

D'fhág tú guta ar lár sna focail sin, Larry:

An tSeirbia, Seirbiach, Seirbis

It's a peculiarity of Irish orthography that you don't find the letter 'e', whether long or short, all by itself except as the last letter of a word: sé, míle, etc. Otherwise, what you get is ei, ea, ae, etc.: eile, cead, tae, éan, péire, etc.

An bhfuil eisceacht ar eolas ag éinne? Bíonn eisceacht ann i gcónaí, nach mbíonn?

Dennis, a chara,

As usual, you are quite correct :-) I was being lazy and, being an Englishman, am not used to the combination of vowels. I answered our friend Sasa using my memory, instead of looking up the translations.

I'm grateful to you for pointing out my error, and I apologise to Sasa for any misleading information.

Le meas,
Larry.

Larry, a chara agus Dennis, a chara

Thank you both so much for your help. I'm very greatful.

Go raibh míle maith agaibh

Sláinte,
Sasa