oh, i see my problem now - i put "na ceapairí" which would be "the sandwiches" not just sandwiches. "tá ceapairí ar an mbiachlár" is indeed correct. mystery solved!
Apparently, both bhiachláir (with an 'i') and mbiachlár are acceptable. Is this a dialect thing? I said "bhiachlár", and that was marked wrong. Thinking about it, bhiachláir sounds like a plural. Is it? Bit confused....
Na ceapairaí means the sandwiches so what you said was the sandwiches on the menu. Tá na ceapairí ar an means the sandwiches are on. Tá means someting is/are. Mar shampla(for example) tá mé ar an mbord = i am on the table