Is it realistic to leave Belfast early in morning, spend day at Valley of Boyne area (Hill of Tara, Newgrange) and catch 6pm flight from Dublin? We will have a car and need to return car at DUB.
You will have plenty of time, Belfast to Dublin Airport is about 90 minutes driving time. Allow an extra hour if brexit creates a border checkpoint between Newry and dundalk.
I looked on google maps and it looks like Belfast to Newgrange is 77 miles so I think if you leave really early it would be doable. We did a day trip out of Dublin to Hill of Tara and Newgrange and if you only have time to see one, I would pick Newgrange. There’s a little museum that takes about an hour to go through and make sure you get on the tour that will take you inside the passage tomb. It’s a pretty amazing experience to be inside that tomb that was built even before the pyramids in Egypt! If you can get to Newgrange by 9 am you should be able to leave that area by noon or 1 pm to get to the airport, return the car and check in to your flight.