We have studied the photodissociation of water molecules at the ice/graphite interface in the presence of submonolayer amounts of potassium. The ice films were grown at cryogenic temperatures and ultrahigh vacuum conditions. They are transparent for 240-900 nm photons, but the strong light absorption in the uppermost layers of the graphite substrate generates energetic charge carriers that may drive photoreactions at the interface. Similar schemes have been demonstrated and investigated before in the monolayer regime, without an ice layer (D. Chakarov, M. Gleeson and B. Kasemo, J. Chem. Phys., 2001, 115, 9477). Here, using ice films of tens of monolayer thickness and with different morphologies, we have investigated the confinement effects due to the ice layer, and the ice permeability for reaction products.