I suspect that you will not be granted Planning. It is a separate dwelling unit and that would be very much frowned upon. Furthermore, if they granted permission, it would allow you to sell that part of your house.
The only way to get Planning for such a thing is to apply for Permission for a Granny Flat.
You'll have to fill out forms, and do a full set of drawings - plans, sections, elevations etc. of the extension, indicating the original and the new bit.
It's a little tricky if you've no experience, so you should probably get a draughtsman etc. - you shouldn't have to pay more than €1200 even in Dublin.
I'd also be a little worried about Building Regs - regarding fire escape for example. Also it might affect your house insurance.