PIR Insulation installation between joists
No, you don't need air gap between the ceiling and the insulation. You do need ventilation of your attic space, however, which unless there is some other form of ventilation you need to have an air gap between the PIR boards and your eaves.

That said, unless you need the space, it would be more cost effective to supplement your existing glass fibre with additional rolls laid across the top (preferably across the joists rather than along them, in order to minimize gaps). That would also remove cold bridges that are currently formed by your joists. If you need boards in the space, you can buy plastic supports that hold the board 170mm above the joist to allow space for insulation.