We took the high speed ferry from Holyhead to Dublin with the Coupe last year. The Ferry was excellent with no loading issues. A few nights in Dublin and then down to Cork and others. Had a great time. We used the main roads rather than the more scenic stuff I am afraid however I did a fair amount of business down there in a previous company and so did travel around.
Assume that as you go further south your average ground speed slows - up to 20%. From moderate speed in the Dublin area you end up following slower traffic in the south and Cork. Also the roads tend to be fine on the main routes but a lot slower off the main paths.
I usually allow for a 20% increase in journey time in the more distant areas. I imagine this may be slightly worse in holiday season.
