The OP was looking for flights on 30 March next year AFAIK Aer Lingus do ORK-MAN from the middle or maybe 25th of March onwards.
Airlines have to ask for volunteers in the case of over-booking, a passenger should never be initially refused boarding if there are volunteers who are willing to give up seats. Volunteers are usually booked on another flight and sometimes compensated at the airlines discretion.
If no volunteers are forthcoming, the airline then must decide who to deny boarding to. If you are dealing with 'professional' desk agents they will sort out those you have onward connecting flights, have a short turnaround time of 1 day or less or those on the outbound leg of a journey. Those on the return leg of a journey will be the ones most likely to be denied boarding. If you are denied boarding and did not volunteer you are automatically entitled to €250 compensation under EU rules (if your flight is under 1500km, more if longer) You must also be provided with alternative means to get to your destination, later flight, other airline flight or taxi etc at no cost to you. If you are in a city where the next available means of transport is the following day you are also entitled to shelter (hotel) for the night.
Know your rights and push desk agents as far as you can epecially those who work for agencies like Servisair who have no ties to any particular airline. I find these agents to be the least helpful. Join your airlines frequent flyer program, even if you don't do a lot of travelling the number will be on your booking and a desk agent will be less likely to bump a frequent flyer.
Bottomline is, if you know its gonna be a busy flight be there early, you'll never be bumped again.