Bear in mind that the ferry crossing always relies on the weather. I have been there in springtime, and there were few crossings due to bad sea *. If the ferry runs while you are there, I suggest to go early morning, so you could stay on the island long enough for your taste. Otherwise, if there are not that many people (due to a downhill tourist season or whatever), you might end up being brought to the island and then told you are given 1 little hour to see it, if the operator wants to finish early. You may count 1:30 - 2 hrs to walk around (wear good shoes - and if cloudy: boots even bettter, cause it gets slippery on the top), more if you want to sit down quietly and enjoy a picnic. Fantastic views over the peninsula on a clear day. I do love this place.
* The tourist office in Dingle is usually advised if the ferry is running or not, so there is no need to drive to Dunquin for the sole purpose of checking that out.