"It is eaten every Saturday."
Translation:Itear gach Satharn é.
The dictionary definition for sé says it right there at the start -
sé sg. m. pron. He; it (usually referring to m. noun). (Subject of verb; not used with copula)
é is not restricted to the grammatical object of verb.
é, sg. m. pron. He, him; it (usually referring to m. noun). (Has various grammatical functions, but cannot be the subject of an active verb)
The verb is in the autonomous form, so there is no separate subject. It is often translated into English in the passive, but it is not really the same. You can think of it as more "One eats (they eat) it every Saturday," but there is a distinct verb form instead of "one eats"/"they eat." In English, "it" is both subject and object, so a different example might help. We say "He was born in Cork" using the passive instead of "His mother bore him in Cork." In Irish you use the autonomous "Rugadh é i gCorcaigh"; the subject is understood. (I think I've got this right.)