The Scheme is actually called the
Northern Ireland Planned Healthcare Scheme and full details can be found at the link.
If you want details on how it works at a practical level, you could consider contacting Michael Collins TD's Dáil Office as he has been using the scheme on a monthly basis for the past few years to get his constituents' cataracts removed in a Belfast Hospital.
(I phoned his office once to make a similar enquiry and he answered the phone himself and was very helpful!)