I've set aside a full day to do Achill Island - you mentioned parts the tourists wouldn't normally see?
I would recommend going to the end to Keem beach. Interesting information board there about an RAF Swordfish plane that ditched in the bay and the crew had to be rescued and then brought up to the border and handed over.
Wild camp up there and Keel also. Also when heading on the main road you will see the transmission tower up on the mountain left hand side, good views from up there.