Nobody here is going to give you advice on walking away from debt in the Czech Republic or anywhere else.
I don't know about that. It all depends on the circumstances....
To the OP:
The short answer is no, not in the short term. It won't affect your credit history in Ireland. The Irish Credit Bureau doesn't have a record of debts with foreign banks.
The problem might be if the Czech bank feels sufficiently annoyed to register a judgement against you there and then tries to get a European Enforcement Order (EEO) or a European Order for Payment (EOP). It would need to know where you are (i.e. your address in Ireland, which mightn't be that difficult for it to find out).
Then again, I can't imagine a bank going through all of this for €20k.
Why are you considering defaulting on the debt? Are you unable, or just unwilling to repay it? Is it secured? I know it is a personal loan, but was it to finance a small business or was it to finance your lifestyle?
All of these will determine whether or not it is a good idea to walk away from the debt, and how likely the Czech bank might be to chase you for it.
Are you a Czech citizen? Have you family, or other roots there? Are you likely to ever live there again?
That too will have an impact on whether or not you should do this.
You might not want to divulge any of this info, but if you come to a public web forum looking for advice, people are bound to ask for more info so that they can help you properly.