This question already has an answer here:
I am looking to go to the USA for a week or so in March and I have just noticed that flights from Dublin (that go via Heathrow) are half the price or less than the same flight directly from Heathrow.
Any idea why this is? I think I will catch a cheap flight to Dublin and add a couple of hours to the journey - I will save over £1000 for the 2 of us!!