Translation:On top of
Can this also mean "at the end (of something)"?
Perhaps as in ar ben yr wythnos "at the end of the week"?
ar ddiwedd y dydd/wythnos/mis, etc is probably more common.
Look up ar ben and pen in the on-line Gweiadur to see examples of how they are used in some useful expressions - click on the 'phrases' tab in the definition. Also look up 'end' to see how it is translated in various contexts.
I think was getting confused with Cornish, where I've seen dhe benn, orth penn (i.e. "i ben, wrth pen") for "at the end of".