Cardiff is your only real option is flying to Wales. Flights from Heathrow,
Gatwick, Birmingham or Manchester
There are a number of smaller airports in the Principality, but it is unlikely
that the tourist would be wanting to use them
You can also consider Birmingham or Manchester as suitable places to fly
in to, and perhaps rent a car
By Sea
Holyhead to Dublin - the Stena HSS from Dun Loaghaire to Holyhead.
Ferries leave every 4-5 hours and take about 99 minutes to cross the Irish
Sea. Alternatively, Irish Ferries run several services daily. Stena
Line web site