Were in the same boat last summer, although not with PTSB and were flat out refused interest only while selling rental property. They deemed our incomes not high enough to meet criteria without the income from the rental. Seemed bonkers really after paying both mortgages, PPR and Rental, for 12 & 20 years respectively without missing a payment ever, even when we were building new PPR and living in what was eventually rental property and having to pay both for a period of time and also having a much higher potential debt to pay when we first applied for mortgage to build the PPR. They seemed quite happy to let us struggle for the six months while selling. Eventually went with a loan from CU to cover costs while selling and paid it off soon as sale of rental was complete.
As I say not PTSB so see what they say. The whole process was a pain as had to submit all docs similar to a new mortgage application. Took them an age to decide, lots of back and forth, and calls not returned for weeks on end. Hopefully a better outcome for you.