This is an anglicized form of the Olde Gaelic name Mac Croimhthainn. The Gaelic prefix "mac" indicates "son (of)", plus the personal nickname Croimhthainn meaning "fox". This name was adopted by a branch of the O'Sullivans whose seat was the castle of Letter near Cahirciveen in Co.Kerry. The name was originally widespread in the Dingle peninsula and on the Blasket Islands. Older anglicized forms of the name found in various Monastic Records include MacCriohin, MacCruhen, MacCriffon etc.. It is interesting to note that the Gaelic prefix "O" (male descendant of) rarely preceeds this name. However the famous Kerry storyteller and author of "An tOileanach" (the Islandman) spelt his name O Criomhthainn. Today the name is spelt MacCrohan, M(a)cCrohon and occasionally MacCrehan. A patent of James 1st (1603 - 1625) instances the chief "MacCrehan alias O'Sullivan" a name meaning "one-eyed". The first recorded spelling of the family name is shown to be that of Mac Croimhthainn of Co. Kerry. which was dated 1596, enlister in the O'Sullivan army. during the reign of Queen Elizabeth I of England, Good Queen Bess 1558 - 1603. Surnames became necessary when governments introduced personal taxation. In England this was known as Poll Tax. Throughout the centuries, surnames in every country have continued to "develop" often leading to astonishing variants of the original spelling.