A barrow is essentially a mound of earth over one or more burials. They are more usually to be dated to the Bronze Age. There are many forms of barrow including ring, bowl, long and bell barrows.
Ring barrows are formed by digging a circular trench or fosse around a central burial, with no mound.
Bowl barrows are formed by heaping up soil over the burial(s) from a surrounding fosse, these often have an external bank too (see Ballyremon Commons (County Wicklow)).
Bell barows are simply round mounds with no fosse or external bank.
Long barrows are rare in Ireland and are more common in southwest England. Their shape is basically ovoid rather than round (see Ballynoe (County Down))
A fosse is a man made trench that is often associated with a bank. Fossse can be used to refer to a dyke, the circular trench around some barrows or the trench dug around a ring fort for defensive purposes.