As it appears, KA-seh, but it's an interesting question, because the name is taken from Dáithí Ó hÓgáin's The Celts: a History but has had the -s chopped off from Casses for some reason. The Roman name is Cassi, which suggests that the people proper might have called themselves 'Castoi'... more discussion on this thread: https://forums.totalwar.org/vb/showthread.php?t=103926