Have moved this post into this forum wgipk as that gives you 2 forums covered ie UK and flights (your uk post is linked to here)

BUS from Belfast eoropa bus depot to Dublin airport hourly during the week takes around 2hrs 15 mins use traveline NI journy planner and put your dates /times in

yasus
wizard

I was also going to suggest the bus but I'd be wary about anything traveline says about anywhere!! Go direct to Ulsterbus info http://www.translink.co.uk/goldlineexpressservice200.asp for times, fares and ticket offices.

wary about anything traveline says about anywhere

Steve i use it all the time never been miss routed or had problems, its like most sites if you don't put in the right info you wont get the right answer

wizard

it might work for this journey but it used to give utter gibberish going to Liverpool Airport. I've just checked again, it does now recognise Liverpol Airport as a destination but still gave the wrong answer. It had me get off the train at one of it's two Liverpool South Parkway rail stations (getting off was right but there is only one station) but then told me to change buses between South Parkway station and the airport - failing to mention that instead of dashing across the station I could have a slow amble for a direct bus that left 15 minutes later but got to the airport only 1 minute later. Which would you rather do with luggage? As far as I'm concerned this is a system for double checking details you already know something about. But we are getting off topic so i'll leave it there!

As Steve says, Ulsterbus the most reliable but there's another service departing from outside Jury's Hotel, next to the Europa bus station, that seems to be very well used, judging by the numbers waiting to get on as I pass twice daily.
Why they didn't think of running spurs from the rail lines that pass close by both Dublin and Belfast International airports, I'll never know, and the longer they tarry, the more it will cost.

The other service is http://www.aircoach.ie/table.routes.belfast.php who are marginally cheaper if you pay in Sterling and marginally more expensive for single tickets bought in Euros. But they are not a 24 hour service so you need to consider whether they can get you to the airport for a morning flight or get you back from an evening landing.

The Irish Government has considered a rail link to the airport and there will be a service into Dublin (eventually) but it's unlikley the traffic from the north will ever justify a main line link. Bear in mind that since the airport terminal already exists the only way to get a train near to it will be in a hugely expensive tunnel. Heathrow has never had a west bound rail link because of this and the study for the proposed British High Speed rail network has already ruled out giving it a loop to provide theoretically faster services to the English regions.

Aircoach... that's the one... saw them loading up again yesterday and noticed they also serve the Dublin city centre hotels

The Belfast-Dublin rail line re-opens at Malahide next week after a bridge collapse last month, necessitating a bus transfer around the works and hitting passenger numbers in a big way... they're offering reduced fares to attract people back... check out http://www.translink.co.uk

Update: Belfast-Dublin rail line re-opened today

Much though I like the train I wouldn't recommend it for travel between Belfast and Dublin Airport, which was the original question of this thread. The direct bus is faster and more frequent.

Hi sorry I did'nt reply to this sooner, we used ulsterbus to get to Dubin airport, it took the bus longer to get there than it did the flight! The fare was £12.30 single or £17.00 return.