If you're planning to go this year and can get your ferry booking in quickly, Stena Line are offering 20% off all bookings made by 21st March (2 days' time)! for crossings taken by 15th December. This is valid on either Holyhead-Dublin or Fishguard-Rosslare crossings and you don't have to use the same crossing for the outward and return journeys. Similar discounts have been offered in January for the past 2 years.
There's no real advantage in using either the Fishguard or Pembroke crossings to Rosslare (other than perhaps timings) - Fishguard is a slightly longer drive followed by a slightly shorter crossing so take your pick.
We took advantage of such an offer last summer to sail from Holyhead into Dublin, spend a few days in Galway (we stayed with friends but there's a caravan site in nearby Salthill), a week in Connemara at the Renvyle Beach caravan park and a few nights in Cahir at the Apple Farm, where we visited Cahir Castle, Michaelstown Caves (I think) and the Rock of Cashel.
We would definitely recommend that trip but just be aware that the road from Galway onto Connemara (I think it's the N59) is in an appalling condition and you'll need to drive VERY slowly over bumps and potholes on some stretches. But boy is it worth the reward - it is possibly one of the most beautiful parts of the world you can see.
I did pen an article to the magazine about it so you might be able to read more one day ...