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!