Greetings from Paradise...
Either way is fine, you'll enjoy either, via Limerick or via the Ferry at Killimer.
However, if you ant my professional opinion, here goes.
I have taken this route possibly more than any Irish person alive and I would never take the Ferry option.
Heres why.
1. You wont even see Limerick anymore if you don't want to, its bypassed by a short tunnel under the Shannon.
2. You'll get to see Bunratty Castle and Adare heritage village.
3. The road is way better. (prone to car sickness? go via Limerick)
4. In my opinion its marginally quicker
5. The Ferry is fun, but not if you miss it and are stuck in the very uneventful parking lot for an hour.
6. The Ferry costs around €15, the Limerick bypass tunnel around €2, thats €13 more for dinner, or Guinness, in Dingle!
7. There is an ugly old power station dominating the entire 20 minute journey across the Shannon.
Whichever way you choose, enjoy, drive safe, and "keep it between the ditches"!
Stephen McPhilemy
Rick Steves Ireland Tour-Guide
Dingle & Derry