Carl, as a person living in Ireland and having invested in Poland, a similar situation to Marcin, I can give you some highlights.
You can get properties much cheaper (e.g. comparing to Dublin) and get a good rental yield. E.g. for buy to let mortgage in Ireland you need 30% downpayment. This 30% for a property in Dublin might be enough to buy a property in Poland in cash. Good cash flow, no debt. In general it might be hard to get financing for such international deals. Also, there is some currency risk, since Ireland and Poland use different currencies.
Apart from that, taxes are a big factor. Ireland has huge income taxes. For me rental profit in Ireland would be taxed at 52% marginal rate (high earner problems...). The rental income in Poland is taxed 8.5% (assuming this income is not remitted to Ireland). Potentially you could use depreciation to lower the Polish rental tax even more. But that requires more paperwork. Also, property taxes in Poland are tiny (think 50 euro a year range). The Irish ones are not very high, ~couple hundred euro a year.
There are lots of smaller and bigger differences. For now in planning to expand in Ireland to take advantage of mortgages.