Landed at Barra about 3 weeks ago. Obviously you need to contact them to find opening times as it's only available at low tide.
The "runways" are quite difficult to spot, but if you fly towards the terminal you can see some marker boards for the runways.
The sand seems very firm when the tide is out and you don't leave much in the way of wheel tracks so your Mooney should be OK. Certainly no worse than if you use it on grass.