Train is not great to connect with Musgrave Park and the last train to Cobh is at 10:30 so you'd most likely have to leave early.
Black Ash Park and Ride would be ideal for you apart from the fact that they have height barriers and no overnight parking. Tramore Valley Park similar but locked gates overnight, both had been mentioned as potential locations for an aire in Cork but nothing has come of that to date which is a pity.
Next best option might be to try somewhere like The Bull McCabes on Forge Hill a call, I may be wrong but I think their primary trade is daytime food and they have a carpark so they might be open to overnight parking if you call in advance and have food there before the gig.
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