If you only have four days, then I suggest you fly to France instead and skip the ferry which will take most of 24 hours plus travel to and from ports. You could spend the extra time in Ireland.
When are you travelling?Edited: 5:55 pm, January 16, 2012
I'd agree with Bean A - it's a lot of time to spend on a ferry for a short trip overall.
As regards the airport to fly into, Dublin makes most sense, as you wish to visit Dublin in addition to Kells. Kells is about 40 miles from Dublin and relatively well served by bus connection from Dublin. You would return to Dublin to travel south to Waterford. You have the option of bus or rail from Dublin to Waterford.
Thereafter, your itinerary would depend on how you travel onwards to France - you could travel by bus to Rosslare or Cork to catch a ferry to France. However in both options there are no direct bus services, so it is quite cumbersome to get from Waterford to either ferry port.
The alternative from flying from either Dublin or Cork to France is probably a better option.
For bus details, see wwwbuseireann.ie and for rail see www.irish rail.ie. For flight options, I suggest you check the websites of Dublin and Cork airports - options will vary depending on your destination airport in France.
There is a direct bus service to Kells, Co. Meath from the airport or city centre, the journey takes a liitle over one hour. (there are no rail routes to Kells). If you need hotel accomodation the Headfort Arms hotel is situated in the middle of the the town. www.headfortarms.ie
Thanks everyone for the helpful ideas.