NIOA and its associated clubs have been searching for a way to present a unified calendar of fixtures. No universally successful way has been found (the search continues).
The most successful approach found to date is “oevents.info”. The “All NI + main GB fixtures” link gives you a schedule of all fixtures in NI, level B fixtures in neighbouring BOF associations and level A fixtures across the UK.
Unfortunately this list is not “mobile-friendly” and the fixture links are to the BOF website (whereas fixture information is usually on the host club’s website). It is, however, a sound source for the schedule as it is driven from BOF’s list of registered fixtures (every fixture has to be registered to be insured). The only time a fixture will be missing is if it’s not been registered with BOF.
For the more adventurous you can select what you want to see in a schedule by using this page on oevents.info.
One rather nice capability of oevents.info is to send you an email each week list upcoming fixtures. You can sign up for the email here.
(An example of a level B event is the CompassSport Cup, an example of a level A event is the “JK”).