Gaels are great ones for borrowing names from other cultures and a good example of this is the three names Seán, Eóin, and Eáin. The etymologies of all three names go back to the middle Latin name Iohannes which is from the older Latin form Joannes, from the Greek Ioannes, and then back to the original Hebrew name Yohanan, in full Y’hohanan, literally, Jehovah has favoured, from hanan meaning he was gracious.
The name Eóin has been in use since anno domini 1100 and is the Gaelic form of the Medieval Latin Iohannes. Seán is the same name but has a different etymology, it is from the French Jehan, which in turn is taken from the Latin Iohannes and this form only became popular in Ireland after the arrival of the Normans. Eáin is a form of Eóin in use among Gaels in the southern Hebrides and Argyll and is found in Ulster where Gallóglaigh and Redshanks settled circa 1400 to 1590s.
All three names are also surnames; the modern forms are Mac Seáin, Mac Eóin, and Mac Eáin. In times past before Gaelic became more (or less) standardized there were several spellings of each of these names, for instance Seán could be found spelled Seaghán and Eóin could be found spelled Eoghunn or Eogháin, this later from especially confusing as it is identical to another Gaelic name spelled the exact same way meaning ‘well born.’
All three of the surnames have a variety of anglicised forms; Mac Seáin is rendered as McShane, Johnson, Johnston, and Jackson. Mac Eóin is rendered as McCown, McKeon, McKeown, McEoin, McOwen, Johnson, and Johnston. Mac Eáin is anglicised as McCain, McKane, McKean, Johnson, and Johnston. Eáin is also found in the surname Mac Giolla Eáin which is anglicised as McClain, McLane, etc.
The variety of anglicised forms of these surnames can provide a significant problem to overcome for family history researchers not to mention all these names were popular and are found in many districts in both Ireland and Scotland, not to mention the Isle of Man. Fortunately, DNA testing easily sorts out one group from the other.