I'm assuming your carbon frame doesn't give you the option of fitting a conventional rack. I'd say you have three options. First, a bar bag. Very useful but you'll struggle to find one large enough for shoes, clothes, tools etc for four days. (If you replace your clipless pedals for cages and straps you can cycle in trainers and save taking extra shoes.)
Second, a saddle bag. http://www.carradice.co.uk/ You can get them bigger than a bar bag but they can be difficult (read impossible) to fit to a racing saddle, so you'd need to look carefully before buying. Third, a beam rack. http://www.evanscycles.com/products/...1?query=Topeak These clip onto the seatpost. They are supposed to be safe up to a 7 kilo load. They aren't as good as a conventional rack because the weight is carried higher, so it raises the centre of gravity and if you were carrying the full 7k would affect the bike's handling, but it sounds as if you're unlikely to haul that sort of weight.

Don't use a rucksack. And you don't mention tools. One of you needs to take some. 400 miles is a long way, even in Ireland, and it can be annoying and waste a lot of time if you aren't equipped even to trim a wheel or fix a chain.

I did the Tour of Ireland challenge http://www.cyclosport.org/eventdetails.aspx?eventid=72 in 2008 and 2009. Great cycling country.