You can hire it from these people:
http://www.landmarktrust.org.uk/pdf/062-063_Clonque.PDF
although, by rumour it is booked up for some time ahead.
The nearest (and only safe) place to leave you rib, would be in Braye harbour. This is about a mile or so from the fort. Apart from taxis, a lift
could be arranged!
Although, be aware, if you are staying there, the Germans, who built the causeway to it, did not take note of spring tides. So in the evening, if you are dining at a local restaurant, the owner may come to your table at say 21.30, cough discretely, and say "if you are staying at Fort Clonque, you might like to settle up and go".