I see Europecar are offering a pre booked secure longterm parking facility now at their hire building next to the the airtport gates. Website quoting €8 p/d, €35 for 7 days compared to the airports €9 p/d rate.
Knock Airport Car Parking | Cheap Car Parking at Knock Airport
NOC often state how competitive their rates are compared to DAA airports, but never mention the private alternatives available at those, good to see some competition at NOC.