This is your unique reference number for all your dealings with the Public Service e.g. applying to the tax office for a tax credits certificate, applying to the Department of Social Protection for a Social Welfare payment scheme, etc.
The Department of Social Protection have responsibility for issuing all Personal Public Service Numbers (PPS No.). Visit that Departments website at http://www.welfare.ie/ for their Information Leaflet SW100 "Personal Public Service Number".
If you do not have a PPS No. and require one, contact your local Social Welfare Office at www.welfare.ie. (Source: Revenue)