For travellers that reside in the UK:

Please try wegolo; wegolo searches all of the major UK and European discount airlines and gives you the results in one page!  No more surfing from site to site comparing.  See what you think below. 

Fly BMI offers flights from Heathrow to Belfast City Airport.  Click the banner below to check them out!

Also see the E-bookers link for flights within the UK to Belfast or Londonderry.                                

For travelers residing outside of Europe:

E-bookers offers international flights which includes their price guarantee on all flight, hotel and travel insurance bookings.  Click for more details and your search. 

Also see the link for Virgin Atlantic as they offer flights from "across the pond" to the UK and Ireland.