Cosy,
If you bought another investment property in Ireland with the remortgaged money then you can offset the mortgage interest against the rental income from the new property. If you have a rental loss (ie interest, expenses & capital allowances exceed rent)on that new property then you can offset that rental loss against your rental profit on your first investment property.
Foreign properties are treated separately. ie you cannot offset a rental loss/s on foreign properties against Irish property rental profits.