Where mountains are ranked by height, the definition of the topographical prominence used to classify the mountain , is noted.

In British definitions, a height of 600 metres is required for a mountain, whereas in Ireland, a lower threshold of 500 metres is sometimes advocated.

The lowest minimum prominence threshold of any definition of an Irish mountain is 15 metres , however most definitions, including the International Climbing and Mountaineering Federation criteria, do not consider prominences below 30 metres as being mountains .

Many British definitions consider a peak with a prominence below 150 metres , as being a top, and not a mountain .

A widely used definition of an Irish mountain requires a minimum prominence of 100 metres , and is the basis for the 100 Highest Irish Mountains.

While Irish mountains are ranked according to Irish classifications, they are also ranked on classifications that cover Britain and Ireland .