Sorry if this is turning into a Knock thread, but we travel once or twice a year from the Midlands to the west of Ireland to visit family. Up until 5 years ago we always went by ferry with the car. I then realised we could travel by air into Knock and hire a car and save money. This is what we've done since.
However we visited at Easter and the cheapest airfare I could find was £500(all in) for 5 returns (looked from Bristol, Luton, Birmingham and E Mids), plus car hire. The fast ferry was £200 return (car and 5 passengers), so that is how we travelled. Now I know that £500 (£100/head) is not a bad fare but we have been spoilt by years of paying £50 to £150 for all 5 of us.
Anecdotal I know but the airlines have to remember that the ferry option is still there. Door to door journey times are 6 hours when flying, 9 hours when driving...not too different.
Dan