We usually require a passenger in each overwing exit row but we do have the discretion, if the pax load is low, to leave the exits free.
As mentioned by others, laws are different from country to country and from aircraft to aircraft, and also dependant on each airlines agreement with their governing body, as total passenger numbers per aircraft and therefore, per exit, can also alter between different airlines operating the same aircraft type.
Rules for the Irish registered airline mentioned are often less strict than CAA registered airlines for all sorts of things.