
There are many historical references to the ancient gaelic variants of the name. One has to bear in mind the many regional gaelic dialects that contribute to the changes in the spelling of the name. An interesting article is held at the New York Public Library that covers 2 variants:
" Sloan, Sloane, Slowan. From Ir. O'Sluaghain, for O'Slaughadain, 'descendant of Slaughadan,' probably a variant of O'Slaughadhaigh, from the MIr. personal name of Sluagadach, 'leader of a military expedition.' The name appears as Slogagadh, the name of a leader of the Bishop of St. Andrew's host, c. 1128. As Slugepah he appears as a witness in King David's letter respecting the consecrationof Robert, Bishop of St. Andrew's at York, 1128. In the charter of Thurstin, Archbishop of York, on the same subject, his name is misspelled Slugedt. An earlier Sluagadach went forth to Rome, c966-971."

Variant spellings: O'Sloane, Sloyan, O'Slowey, Sloane, Sloyan or Sloyne (a rare Mayo name) & Sloane (of east Ulster) are both stated by Woulfe to be anglicized forms of O' Sluaghain, an abbreviated form of O'Sluaghadhain, which is derived primarily from the Irish word slaugh, a host, legion. Woulfe further suggests that it is a variant of O'Sluaghadhaigh (Slowey) of west Ulster, which is now a rare surname but was of importance in early mediaeval times. In north Connacht, Maelpatrick O'Slowey, who died in 1015, is described by the Four Masters as "sage of Ireland". After 1200 references to both these names are very infrequent until the seventeenth century: from there on Sloane is closely connected with Co. Down. In 1659 the census enumerators found Slowan one of the most numerous Irish names in the barony of Newry, while one Alexander Sloane is returned as a titulado in the barony of Kinelarty. Slowey has become Molloy in counties Cavan and Monaghan though the elision of the C in Mac and aspiration of the S.

