Hi Ian!
It's so exciting that you're thinking of exploring the
Boyne Valley from Dublin. :) I did a search for half day tours of Newgrange, and I have found that
Day Tours Unplugged offer this option!
Take a look here for more information, as well as their contact details. If you reach out to them directly, they will gladly tell you more about the private tour options.
If you like history, I think you might also enjoy the rest of
Ireland's Ancient East. Feel free to reach out here at any time if you have any questions!