What does drum mean in place names?

Drum or Doon or Dun refers to an elevation or rounded summit of a low hill often fortified, possibly also applied where such elevations stand out in the topography (for example Dunshaughlin is the fort (Lissen) of the flatlands.) It sometimes applies also to rounded peaks in hills.

Dun can refer broadly to any kind of enclosed fort in Ireland - from Neolithic passage tomb or stone ringfort from the bronze age or iron age. The term Dún in the plural gives its name to County Down.