Evaluate my itinerary? Dublin to Cork

My 2 Euro cents:
Driving in central Dublin isn't for the faint of heart and parking can be problemmatic. Suggest saving a day's car rental cost by just picking up your vehicle at the airport the morning of 11th on your way to Newgrange.
Flying over to Inishmore would resolve your concerns about sea sickness on the ferry and would add a scenic flight into the mix. Though staying on the island overnight is a delight, you can pretty much see what there is to see in a day ... especially if you've saved the time a ferry crossing would entail.
The Skellig Ring Road is a very pleasant diversion off of the ROK, not least because the tour buses don't go out there. You'll be treated to picture postcard views of the Skelligs, a stop at the Kerry Cliffs, and the ruins of an old castle near the village of Ballinskelligs - where there's a nice stretch of sandy beach if you fancy a picnic.
On your way to Cork on the 14th: the Beara peninsula just south of Kenmare is a favorite - lots of pretty hikes and terrific scenery without the crowds of the ROK.

By the time you get the car and get into the city, find parking, etc., you won't have time to see much in Dublin. Why not leave it until next time? I would book Newgrange for the afternoon of your arrival and stay in that area, maybe Drogheda or Slane or Trim? Then head west the next day. This will give you an extra day, which you could add to the SW or stay a night in Athlone to see the castle and old pub there.

You don't have time in your itinerary for a visit to teh Aran Islands.

Don't forget when estimating driving times to allow for time to park up and get to the attraction and then back to the car. This can easily add another 30 minutes plus to times.

The Dublin to Galway day is ambitious. Looking at the Newgrange website it states
Brú na Bóinne tour + Newgrange Chamber will become the main tour option. This includes Nowth and takes three hours. Newgrange Tour + Exhibition (does not include Nowth) ) will become a limited tour option only available on select dates in the morning. This takes two hours.

Allowing for time to get parked, arrive at the visitor centre and then back to tthe car, you are looking at more like 3-4 hours - and you will need to factor in lunch too. Clonmacnoise also deserves and hour to visit.

Cliffs of Moher - the trail south is closed so you can't walk this stretch, although it sounds as if you can walk the stretch to the north. Cliffs of Moher are always busy.. You stand at the top with the world and his wife. The best views are actually from the sea. Having said this, the drive through the Burren is very well worth while and the limestone scenery is completely different to the rest of Ireland.

Killarney to Cork, head down through Kenmare to Bantry (possible stop Bantry House and Gardens) and then round the coast through Skibbereen to Kinsale (stop and explore its an attractive town and there is also Charles Fort too) and end up in Cork. The drive without any stops will probably take 4-5 hours.

When we toured New grange the actual bus ride and chamber tour were later than our timed ticket. Maybe 45 minutes? an hour?

It seems they build in time before your bus for you to see the visitor center (which is very well done). We only did the main chamber at new grange and 2 hours was a reasonable amount of time. The distance from the parking area is not far. It's a very well-oiled machine that runs well.

We headed there from airport and it was an easy drive - we also visited battle of the Boyne that same day before heading North.

Thanks for all the great input! Definitely helps a lot!

FYI, the rental car offices at the airport open very early - 5:30 AM in some cases, so you could get an early start and be at Newgrange when the Visitor Center opens (and the tours start) at 9:00 AM.
Also, the rental car lot is located along Swords Road about a half mile from the airport terminal, so you'll want to alert your taxi driver that's where you're going to preclude having to double back.