Greetings from Dingle,
If you book the ferry online theres two positives
1. You'll save a few euro
2. If theres a a long line of cars ahead of you, you'll be guaranteed to get on, many have to wait to the next ferry sometimes because its so full.
I've taken this very route about 300 times and personally I'd never take the ferry.
- The road is bumpy, travel sick passengers can be a downer
- the view is arguably nice but the two big ugly and dominant power stations that burn coal and oil there morning, non and night are far from scenic
If you go via Limerick you'll enjoy a better road, get there at the same time if not quicker. You'll also see the heritage village of Adare, visit the Norman museum, light a candle in a 12th century monastery and have a coffee in a thatched cottage - all on the little main street! You'll also pass Bunratty Castle, possibly worth a visit, definitely for a photostop.
Either way you can go over the Conor Pass or go via tralee, its only after Tralee do you have to make the choice - Ferry to Limerick, hmmm??? :)
Enjoy.
Stephen McPhilemy
Rick Steves Ireland Tour-Guide
Dingle, Derry and Dublin