Knock is reasonably busy Ryanair fly to/from there from various destinations as well as other airlines.

Last year I got a very cheap flight back from there to Stansted at the end of June, fare was 0.99 euro and taxes and charges came to 6.60 euro so in total the flight was 7.59 euro BUT beware. Although Ryanair collect the 6.60 taxes/charges up front when you book, Knock Airport wouldn't let anyone check in unless they paid another 10 euros "development" fee to the Airport direct.

Not sure if this is still the case this year but bear it in mind.

Mind you at the price I paid it was still cheaper that the train from Stansted Airport back into London! Unless you know the cheap, perfectly legal loopholes for that journey which reduces it considerably [consultancy advise always given at a price ;-)]