It's of very little relevance where you got married. Eg, if you got married in Vegas, you wouldn't go back there for the divorce!
Generally, jurisdiction goes on residence, and if you're both resident in Ireland for 20 years, that would pretty much settle it. Also if there's property in Ireland, that's another good reason for doing things here. And if you have Irish resident children, then it pretty much has to be Ireland, as other countries' courts won't want to get involved.
To get divorced in another country, one or both of you would probably need to establish residence there. Generally, when you start family law proceedings in one country, and jurisdiction is accepted there, you can't then switch to another country to finish things off.
However, to echo
@Paul M. it is indeed complicated and you need to speak to a family law solicitor ASAP.
(There is also the issue that some jurisdictions are more or less favourable to the economically stronger or weaker spouse!!)