So, no there is no priority in times of IRROPS with EJ+, the best way to manage your disruption is with eJ app which lets you rebook flight, apply for refund and book hotel room for the night. If the flight is full they will not put you on SBY, sometimes flights are overbooked but this is more due to commercial decission then airports managing IRROPS.
eJ+ will allow you access to priority check in, not sure whether priority check in still applies at the times of IRROPS, but I would probably try and go there first before joining any queues for sales desk.

I do think eJ+ is really good value for money tho, considering that front row seats are going at £17+ and you get fast track + extra piece of cabin baggage. And let's not forget free transfer to earlier flight on the day.

I did fly into STN on Sunday with EZY, flight was 4 hours late but at least it did operate, arrivals were lovely and quiet, once landside it was utter chaos with queues snaking everywhere, took us 15 minutes to leave the building.