Cool place to overnight within 45-50 minutes from dublin airport @end of trip

when you say cool, I assume you mean in the style/fashion sense rather than temperature?

Two places come to mind - either Howth or Malahide?

Correct. Temperature doesn't matter. :^)

Any of those coastal towns south of Drogheda is probably fine. Portmarnock has a golf club with a spa to help you recover from your long drive back from Dingle. Less than 30 minutes to the airport. https://www.portmarnock.com/spa/

Trim is about 45 minutes away and has the Trim Castle, a monastic ruin, (complete with sheep wandering around), the Yellow Steeple and a stone bridge over the River Boyne. We had a pretty good Indian meal there. I don't remember the name, though. Malahide has some great restaurants and a manor house with gardens.

Malahide was nice. A short taxi ride from the Dublin airport. We toured the castle. Pretty interesting tour. Then we walked to town and ate a good meal in a pub. It would be a very short drive. Nice waterfront marina.

We visited Newgrange and Knowth burial sights on our way from Belfast to Dublin Airport. This amazing sight is about 30 minutes from the Dublin airport. I do remember seeing some accommodations nearby.

I second the suggestion for Malahide. It has a beautiful castle and tour (maybe 30 mins long) with gardens attached and Avoca, the cafe next door has amazing food plus it's not too far from Dublin city.

I would say either Malahide or Howth. Both wonderful seaside towns where you could easily fill your last day.

Thanks all for the suggestions. This is exactly the type of information I was looking for. Thank you!