If you mean the Republic of Ireland then no, we are in Euros here, if you buy a postal order it will be in Euros and can only be cashed within Ireland so if you send it to someone in the UK it will be useless as the person there will not be able to cash it.
You cannot even send an Irish Euro postal order to France or any other Eurozone country, which I think is ridiculous but that is how it is, an Irish postal order can only be cashed in an Irish post office.
If you mean Northern Ireland then the postal order you purchase will be Stirling the same as the rest of the UK and can be cashed in any UK post office be it Belfast, Nottingham or Edinburgh

