The standard pitch on Ryanair is 28", the same as on other European charter operators for a couple of generations now.
The exceptions are row 1, at the door, and the two rows amidships alongside the two overwing emergency exits. However you state you have injured your knee, and the exit row seats are for able-bodied passengers only. Therefore your main chance is row 1. However if the flight is less than full the front and rear rows are closed off to maintain the correct balance of the aircraft, as otherwise with free seating you can e an imbalance.
The routes from Ireland to Poland have taken more than a bit of a commercial downturn in recent times, so it's unlikely to be full. Ask checkin how many passengers are booked for the flight, and unless it's nearly the full 189 then the front will be closed off. Best bet then is to board at the back and sit in the rear row, these seats normally fill up last if at all.