The Irish authorities don't require a passport but the airline will require appropriate ID. What counts as 'appropriate' varies greatly between airlines. The only operator between Cardiff and Dublin appears to be Aer Arann. Their website isn't particularly helpful, as it splits travel into 'domestic, within the UK' and 'travel to/from continental Europe'. I suspect that the rules for ''domestic, within the UK' apply to travel to Dublin. (Before any infuriated Irishmen post a rely, I'll acknowledge that Dublin is most definitely not in the UK!). That view is based upon the reference, on Aer Arann's website, which applies the same rules to children travelling between Ireland and the UK as to domestic flights.
If I've interpreted Aer Arann's rules correctly, you need a passport, driving licence (with photo), student card, bus pass, disability card or work ID (with photo):
http://www.aerarann.com/about_us/passenger_inf ormation.htm
However, I'd recommend checking with Aer Arann before booking. You can email them here:
[email protected]
or phone them on
0870 876 76 76
(Alternative number: 00 353 1 844 7667, M-F 0900-1700)
Chris