If you cease to be an owner occupier and rent the property out then it becomes a rental/investment property with all that that entails - e.g.:
http://www.revenue.ie/en/tax/it/leaflets/it70.html
In fact even if you don't rent it out but cease to be an owner occupier then I believe that you will at least be subject to the second home property tax. Whoever told you that the property would only be classed as a rental property if you took out another mortgage was wrong.