Belfast to Donegal

Sounds likes a bit of a long drive to me. We did Ardara (we stayed at the Green Gate as well) to Buncrana and that was a moderate day in mid-summer. GC is interesting, but we found it way too touristy for our liking. We enjoyed the cliff walk more than the Causeway
Sunset at the Green Gate B&B was amazing, but it's probably setting behind the hills now instead of over the ocean. Regardless, you want to get ther early enough to enjoy it.
There's also a thread about the Green Gate a few pages back you might want to read up on.