It would be normal for lenders to ask for recent (typically, the last month's) payslips at drawdown time to confirm nothing had changed since the loan approval was issued. If that can't be supplied, you'd be asking them to reconsider the loan approval in view of circumstances excluding an income previously relied upon to get loan approval.
I would say PTSB, like other lenders, go through a checklist of supporting documentation to cover themselves in terms of their loan underwriting and payslips would be part of this list. Do you think you would make the income level for the previously-approved loan amount now that the firefighter role is gone?