Assuming you have a dingy then some info is here, including a phone number to get a mooring-
Arran Yacht Club - About Us
I have seen people launch boats on the slip but no idea of cost, I'm sure the phone numbers on the website would know though.
Another option maybe using loch ranza, better base to get to Kyle's, loch fyne or campbeltown, some info here
https://www.harbourguides.com/harbou..._Arran/marinas
If no joy then the lochranza hotel worth calling to ask as the lochranza website doesn't work, the phone numbers might though. You would need to find out about launching at lochranza, I assume the ferry pier is what the locals use but worth asking.