The regions, with their names and numbering are taken from The Relative Hills of Britain by Alan Dawson.
A Marilyn is "a hill of any height with a drop of 150 metres (nearly 500 ft) or more on all sides". So it is a hill which is relatively high compared to its surroundings.
A subMarilyn is a hill which just fails (by up to 10m) to qualify on the drop rule, i.e. drop of 140m to 149m on all sides.
The Marilyns are so-called by the list's compiler, Alan Dawson, after the more famous mountain list - the Munros. Alan's original list is of mountains and hills in Scotland, England, Wales and the Isle of Man. The same concept was applied to Ireland by E D "Clem" Clements
(Marilyns appear as Class=Ma on this website, and subMarilyns as Class=sMa).