A streamlined mound of glacial drift (7)
I believe the answer is:
drumlin
(Other definitions for drumlin that I've seen before include "'Small ridge, common in Cavan and Monaghan (7)'" , "Hummock of boulder clay left by a retreating glacier" , "oval mound" , "Small elongated oval hill, in Cavan, say" , "Small hill, common in Cavan and Monaghan" .)