If its just ordinary grass not couch. I would be tempted to do no dig, Put cardboard directly on the grass, soak it cover with compost or manure to 8" - 10" thick. When you plant your potatoes, move the mulch, cut a cross in the cardboard and remove the top layer of gras just where you want to plant.
Take out a 3" hole, drop tattie in cover with the soil removed, draw cardboard back over and pull the mulch over as well. Through the season you can top the mulch with a thinish layer of grass cuttings each time. This helps to conserve moisture and exclude more light.
It really does work very well, as the worms move around more freely they pull down the rotting cardboard and mulch. When you come to digging up the crop its surprisingly easy and friable.
I had trouble believing all this until I tried it................specially good for anyone with back problems - or lacking stamina like me ;)