In the Irish language, the presence and placement of the síneadh fada is significant, as it changes the meaning of the name.

As Irish has no letter ⟨j⟩ (derived from ⟨i⟩; English also lacked ⟨j⟩ until the late 17th Century, with John previously been spelt Iohn) so it is substituted by ⟨s⟩, as was the normal Gaelic practice for adapting Biblical names that contain ⟨j⟩ in other languages (Sine/Siobhàn for Joan/Jane/Anne/Anna; Seonaid/Sinéad for Janet; Seumas/Séamus for James; Seosamh/Seòsaidh for Joseph, etc.).

The name was once the common equivalent of John in Ireland and Gaelic-speaking areas of Scotland, but has been supplanted by a vulgarization of its address form: Iain or Ian.

[5] When addressing someone named Seán in Irish, it becomes a Sheáin [ə ˈçaːnʲ], and in Scotland was generally adapted into Scots and Highland English as Eathain, Eoin, Iain, and Ian (John has traditionally been more commonly used in the Scots-speaking Lowlands than any form of Seán).

Even in Highland areas where Gaelic is still spoken, these anglicisations are now more common than Seán or Seathan, undoubtedly due in part to registrars in the United Kingdom of Great Britain and Ireland and the United Kingdom of Great Britain and Northern Ireland having long been instructed not to register Irish or Gaelic names in birth or baptismal registrations.