Aer Lingus is currently running a sale from the UK to Ireland, offering extremely cheap flights from a number of UK to a number of Irish airports. In a number of cases, it is actually keeping costs to a bare minimum by charging nothing for the flight fare.
The link to the promotional page is here.
The sting in the tail here, of course (and I must apologise for my slightly sensationalist headline), is that you still need to pay the taxes and fees.
However, with taxes as low as £12.99 each way, this still leaves you with a number of budget options. Factoring in the £14 admin fee, the lowest return flights we could find, with the “free” base fare, were £39.98.
For example, there are currently a number of £39.98 return fares between Birmingham and Dublin:
(You can see in this picture that the “fare” is actually “0.00”)
Slightly oddly, given that the fare is often nothing, Aer Lingus seem to be marketing this promotion as “25% off your fare”. While they seem to be doing themselves a disservice with this description, it does appear that where you are charged for the fare itself, you will get 25% off that.
You need to book your flights by 4 August, and fly between September and November 2014.