I haven't decided as the final destination is equidistant - probably Belfast as I haven't been there before and pound sterling.
Thanks for the offer.
Your final destination is equidistant from both Belfast and Dublin? If so, I'd chose Dublin if you're cycling. The old Dublin road route is great for cycling as the carriageway has taken the majority of traffic away. The 10 or so miles to Belfast International Airport (which is nowhere near Belfast) is a very busy road and I wouldn't be cycling it if it could be avoided. If you are set on Belfast, you could get the train to Antrim (Translink trains allow bikes on the 1st and last carriages) and ride the back way to the airport, which is much quieter.