An incident like that should not happen at any modern airport,regardless of the airline involved.
Very strict standard worldwide procedures are in place for boarding or offloading passengers on stands with other aircraft adjacent or nearby.
Firstly the ground crew on both aircraft should have been aware of each others status.
Second;the crew of the aircraft should not have requested,nor been given pushback clearance from ATC (or the apron controller),until the other aircraft had finished boarding.
The headset person (and groundcrew),on the aircraft pushing back,should have held the aircraft in position until the other had finished boarding;and also should have reminded the flight crew that the adjacent aircraft was still boarding.
This incident does not reflect well on Ryanair,and shows yet again the pressures their crews are placed under by management to achieve fast turn arounds;even if it involves disregarding standard and basic safety procedures.
