It reads like it means the differential pressure, which is the only gauge marked in psi.
The previous paragraph says:
"On downwind (approximately 3,500ft) to RWY 17 at EICK, it was observed by both flight crew members that the aircraft had started to pressurise, and the differential pressure between the aircraft altitude and the cabin altitude was increasing."
Also the crew were in Manual AC (CPCS aircraft) with the packs now ON and presumably still with the outflow valve closed from the mask drop. So it is possible that they landed pressurised. The FLT/GND switch would have safely dumped the pressure before the doors opened.
The report is at
http://www.gov.ie/tec/aaiu/2001Repor...4/2001-014.htm