As I recall, "dun" refers to a hilltop fort and I suspect that "esky" is closely related to "uisga meaning water. A quick wander with Mr. Google found references that may be helpful in finding possible locations.

"The Esky goes back to An easca, meaning the sedgy bog or marsh. This name is now applied locally only to a hill in Derryloiste which rises boldly up from the surrounding marsh-land. Evidently, it was transferred from the marsh to the hill, after its meaning had been forgotten." http://www.craigavonhistoricalsociety.org.uk/rev/turkingtonmontiaghs.html

(Name of Mooney) "...The Connacht family were located in the parish of Easky in the barony of Tireragh in Co. Sligo..." and "...The Sligo family gave their name to the townland of Ballymeeny in Esky parish." http://www.eskermore.com/history.html