This rings a bell - one of the difficulties the Riverdance probably had was a shift of cargo, creating a list that they could not counteract with ballast. The early photos of the Riverdance clearly show that the cargo on the upper deck had shifted.
This design of ro-ro - accommodation aft, with an open upper deck - does expose the cargo to the weather and the loss of trailers over the side is not uncommon. However, it could be said that this is a good thing - if in horrendous weather, the cargo shifts severely, it is lost over the side rather than ending up piled up against a bulwark causing the ship to list.
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Niall Kelly
http://shipphotos.bradda.com