Current year TRS back payments are, I believe, usually dealt with by the institutions. Previous years are dealt with direct from Revenue.
Be careful, some institutions give this in one lump sum, but I think from other threads here that some people have encountered situations where it has been paid of the remainder of the year. If you don't soon see a once-off large lodgement or large once-off reduction in the net mortgage payment, I'd ask PTSB for clarification on the back payment.