The times also make rugby fixtures problematic (although the world cup matches are on Sunday and should be OK). I would have imagined rugby fans constitute a not insignificant part of the Munster-South Wales market.
Nevertheless, I would imagine that the aircraft is in use on days that better suit leisure passengers. There are loads of routes out there that could be made to pay operationally on a Friday and Sunday/Monday, but it would leave aircraft sitting round on other days incurring depreciation costs while not earning revenue.