There's a bit more to that story, although I don't know that it relates directly to Paddy Zulu or one of the other two Aer Lingus 747s on which BA were conducting a major check.
Apparently, the "Fly Green Side Up" placard was translated into Gaelic and left on the centre console, unattached to the aircraft, as a bit of light amusement.
Before the arrival of the Aer Lingus crew to take the aircraft home, one of the BA senior engineers did a walk-through of the aircraft, and spotted said placard on the the flightdeck. Not appreciating its content, he thought it must have been a mistake that it had not been mounted properly, so called two mechanics to screw the placard onto the aircraft in a blank mounting space.
They duly did as tasked, and an apoplectic reaction ensued from the Aer Lingus crew when they arrived and saw the placard. It was, I'm told from someone who was present, nearly cause for deployment of a UN peacekeeping force to the BA hangar at Heathrow.
I'm sure the tale will have been embellished somewhat over the years, but that's what I heard from someone who said he was there....