How do I find out where my Irish ancestors were from?

I don't think there is an easy way to make progress so I'm looking forward to hearing what others have to say! At least you have a county - my gt*3 grandfather joined the army in Ireland giving a place of birth where no family of that surname lived and then 10 years later turns up in India - married to an Irish girl - and they never give their birth county in any subsequent census records - just Ireland! Thanks for your help gt gt gt grandad!!!