Contact the High Court office in the Four Courts- they will talk you through it. Or your local solicitor will be able to draft up the necessary affadavits and have it dealt with in Court on your behalf ( through law agents) for a modest fee.
Just so you are aware of all the options you don't necessarily have to go the Deed Poll route. 'Common usage' allows you to call yourself anything you like and there are straightforward procedures to have that name used also by all relevant parties.
Treoir is an organisation that specialises in this and many other issues affecting unmarried parents and their children. See the website at http://www.treoir.ie/ for contact details.