Just so that I understand: Brú na Bóinne Visitor Centre is not accepting advanced reservations. It's first come, first serve on any given day. Two of us are planning to be there on 27 June 2019 and we intend to get there well before 9:00am to be sure we have as early admission as possible. Are both Newgrange and Knowth doable in one day? This will be our one and done chance of a day to do this. Thanks for the help...
After breakfast and leaving our B&B in Trim we drove to the Brú na Bóinne Visitor Centre. We saw Newgrange around 10 -11 a.m. and Knowth in the p.m. Since visitors are bused to & from the sites you don't get to control how long you stay, but we felt like we had plenty of time at each site.
If you get there early, you can do it. The trick is to get the first out of the way without a long wait. To do that, you need to beat the crowds in the morning.
We checked out of our B&B in Dublin after breakfast and drove to New Grange. We had plenty of time to do both Knowth and New Grange, tour the Visitors' Center (where we were able to grab a bite to eat) and drive on to Belfast--where we checked in to our B&B by tea time.
Be aware that the visitor centre is currently closed for refurbishment. The shuttle buses to Newgrange and Knowth are still available of course. They can both easily be done in one day. You will miss the exhibits and audio-visual display at the visitor centre (and the restaurant 😀) but I am sure you will still enjoy your visit.
Thank you for the heads up about the visitors center. We will be there in July and hadn't checked that far ahead. So now we can decide if to leave it for another time or not.