This is an anglicized from of the Gaelic (Irish) name O'Cathalain. The Gaelic prefix "O" indicates "male descendant of", plus the personal name Cath(g)alain - a compond of the elements "cath", a battle, "gal", brave and the diminutive suffix "an". There were three distinct septs of this family in Ireland. The most important belonged to the ancient territory of Oriel (comprising the modern counties of Armagh, Monaghan and parts of South Down, Louth and Fermanagh), Co. Limerick was the orginal homeland of the Munster sept. Here, the usual anglicization of the name is Culhane and Cahalane in Cork and Kerry. A third sept was located in Co. Roscommon. An interesting namebearer was Fr. Bernard Callan (Brian O'Cathalain) - 1750 to 1804, a noted Gaelic poet and scholar. The first recorded spelling of the family name is shown to be that of Cathalan, King of Farney. which was dated 1028, in Salin. during the reign of High Kings of Ireland "with opposition", 1022 - 1166. 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.