Marnus is a product of the Toyota Cheetah Academy. He made his GuinnessPRO14 debut on the 8th of September 2018 against Ospreys in Swansea.
He played his first game in the starting line-up on Saturday against Connacht in Galway.
Van der Merwe made his first starting appearance for the Toyota Cheetahs against Connacht in Galway.