Never been to Donegal but want to make it there this fall. Is it best to fly into Dublin? We are not looking for the fastest necessarily. We prefer flying in & out of Shannon, picking up car at airport, maybe spend first night in Doolin, then head to Donegal for a couple of nights. Then onto Dingle.
What are must to see in Donegal?
I would suggest flying Open Jaw/Multi City with Aer Lingus-
one way into Shannon, the other way Donegal via Dublin.
Aer Lingus have morning and evening flights between Dublin and Donegal, so it would be whichever worked best timing wise.
Then one way car rental Donegal to Shannon or vice versa.
We spent time in Donegal on our 2015 Northern Ireland trip but not by direct flight. It was part of a larger trip. Train from London to Belfast, then train to Derry, then car into Donegal on a long day trip. Glenveagh National Park, Bloody Foreland area, Dunfanaghy, Horn Head, Killahoey Beach, Portnablagh, The Mill Inn (not open for season when we visited).
In 1997, I visited Donegal. After landing in Shannon, I picked up a rental car in Galway and drove all the way to Letterkenny, Donegal. It was definitely worth it.
I loved visiting Glenveagh National Park in Donegal: https://www.nationalparks.ie/glenveagh/.
On the way to Donegal, I loved visiting the Ceide Fields in north County Mayo: https://ceidefields.com/. It's a fascinating archeological site, and the views of the ocean there are better than those of the well-trod Cliffs of Moher in County Clare.
Also on the way to Donegal, I heard some amazing Irish music in the Tubbercurry and Ballymote areas of County Sligo (https://thesession.org/sessions/search?q=The+Tubbercurry&format=rss&page=1) where locals are playing mostly for locals. Tubbercurry also hosts an Irish music school.