Michele, Since we are doing the Inishowen Peninsula in Donegal, I wondered if we would have enough time to see Glenveagh Castle and Gardens on the drive from Killybegs to Letterkenny. If we were doing this, we would skip the Fanad Head peninsula this trip.
It will depend on your route. If you had planned on doing the coastline then I would not try to see Glenveagh. I did it a few months ago going from Ardara to Glenveagh through the Glengesh Pass. I had plenty of time for Glenveagh, which was just fabulous. I think the weather will play into the picture. If the weather is dreadful you will want to skip Glenveagh. If you decide to visit Glenveagh take the bus to the castle and tour it. Then walk through the various gardens and trails. You will need at least 3 hours to do it any justice.
The quickest way is to go back into Donegal town and take the N15 & N13 north to Letterkenny. Then take the N56 & R255 to Glenveagh. If Letterkenny is your overnight stop you would need to backtrack to there.
A longer more scenic way is from Killybegs north to Ardara, Dungloe & Gweedore. Then take the R251 east to Glenveagh.
I did something like the latter route except my starting point was Ardara. Either one will work.