When we planned to move at Ireland it was to register as self-employed and so we can legal working in this country. It has several steps and I think is good write about them to help who anyone need it. Here we go!
At first, We both, (Paola and me) are Software Engineers, so this steps are to work like as. I don't know the steps for others proffesions, anyway, maybe will be similar.
First of all, you need a PPS Number (Personal Public Service Number).
This number is like a ID that identificate you and where you are living. You can get it in a Welfare offices of Ireland.
For this, you need a proof of ID (Passport, National ID, Driving Licence,...) and a evidence of living in a irish address, (like household utility bill, official letter/document, financial statement, property lease or tenancy agreement, verified employers letter. This is necessary because Irish administration checks the truthfulness of your address sends you a postal mail to your irish address. It takes 1 week more or less.
So, as you guess, you need living in Ireland. You can rent a room in a flat/house, or B&B/Hostel. Thanks a God, Ireland is a cheap place to living. You can find 2/3 bedroom flats for €800/€900 per month (€260 for room).