As LowCO2 says, you definitely need planning permission for this.
Some Councils, especially in Dublin, are usually quite open to this idea however they will only consider granting permission where each of the proposed houses meets the minimum criteria/standards for a new house - usually standards as set down in the County/City Devleopment Plan - criteria like floor areas, garden/private open spaces areas, off-street parking standards, etc. This usually means that the original (single) house and site has to be quite big to start with.
Alterations to the exitsing house will be pretty intrusive as you will need to seperate the proposed units/houses with fire seperating construction to comply with Building Regulations. In addition, as you would be creating a 'new' house (and altering an existing) you will (probably) have to comply with higher insulation standards to comply with Building Regulations.