The surname Scanlan was first found in County Louth (Irish: Lú) the smallest county in Ireland, located on the East coast, in the Province of Leinster. "The Mac Scanlans were a sept of Louth, from whom the ancient locality of Bally Mac Scanlan took its name. The first of the family mentioned by D'Alton is Patrick O'Scanlan, who was bishop of Raphoe and afterwards archbishop of Armagh in 1261. " According to O'Hart, the family claim descent from the Heremon Kings of Ireland, more specifically through the O'Shaughnessey pedigree as Tuadan, brother of Duach was the ancestor of O'Scannla which is Anglicized as Scanlan. However, MacLysaght claims that there are two quite distinct septs; the O Scannlain of Munster and MacScannlain of Louth. The latter claimed Bally Mac Scanlan (Ballymacscanlan) as their ancient homeland. He continues pointing out that there are six places names Ballymacscanlan throughout Ireland eluding to the family's widespread distribution.
The variant of this name is O'Scannell. There are at least two quite distinct Septs whose descendants are now known as Scanlan or Scanlon. One is O'Scannlain of Munster and the other is McScannlain of Oriel, Louth, neither of which has retained the prefix O or Mc in modern times. The latter are perpetuated in the placename Ballymascanlon near Dundalk. The widespread distribution of these names is indicated by the fact that there are six Ballyscanlans in Ireland as well as a Scanlansland and a Scanlan's Island. Two of these are in County Clare and one in Mayo, which lends colour to the statement that there was also a North Connacht Sept of O'Scanlan. In Ireland the name is chiefly associated with a most tragic event, the Scanlan murder in County Limerick in 1819, which was the theme of several novels and plays, the best known of which is 'The Colleen Bawn'.