Yes. Each vacancy must be advertised to allow your parishioners to decide whether or not to demand a public election. There is a prescribed timetable for this, allowing electors 14 days to submit their petition after the notice has been published, so no further action may be taken until the 14 days have elapsed.
If no poll is demanded, there is no specific legislation for the process of co-option, other than the requirement to fill vacancies at the earliest opportunity.