 The townland covers 202 statute acres.There was a medieval corn-mill on the River Blackwater, from which the townland derives its name.The earliest known mention of Killywillin is in the Irish annals for 1495 A.D. concerning the death of the McGovern clan chief, Feidhlimidh Mág Samhradháin, the First, who ruled from 1478 to 15 Feb 1495.