Since they cancelled the 1110 mid morning flight they have had to rebook those passengers. Unfortunately that means massive overbooking is with us for at least a few weeks, especially with the NI holiday season kicking off from wednesday onwards.

Looking at what you have said about this instance I am not sure when they actually told the flight was overbooked. If they told you before you paid for the FE seat then they were likely giving you a seat under the guaranteed seat scheme for Gold card holders (if you booked more than 48hrs before). In that case, while they sell you an FE at full fare, they have the option to seat you anywhere.

I would complain about the lack of service but dependent on the specifics of the booking its unlikely you get a refund.