Over time some plurals have changed, just to add to the complexity of English, eg the plural of roof used to be rooves but now roofs is accepted, ditto hoof.
sanmac - TTT was pointing out that many people say "a dice", not "a die", hence treating dice as a singular word when it is a plural. "Data" is another example where people think it is a singular word when, in fact, "datum" is the singular.
lady-j, it used to be singular as in pease pudding (which is singular, like apple tart). Eventually people used to hear pease as a plural because of the S on the end, so they started to use pea as the singular.
Cerise is still singular in French as cherise used to be in English.