No need to go back so far.
Just 500or 600 years!
Classical Gaelic, otherwise known as Early Modern Irish, covers the period from the 13th to the 18th century, during which time it was used as a literary standard in Ireland and Scotland. This is often called Classical Irish, while Ethnologue gives the name "Hiberno-Scottish Gaelic" to this standardised written language. As long as this written language was the norm, Ireland was considered the Gaelic homeland to the Scottish literati.
Later orthographic divergence has resulted in standardised pluricentristic orthographies. Manx orthography, which was introduced in the 16th and 17th centuries, was based loosely on English and Welsh orthography, and so never formed part of this literary standard.
From Wikipedia:
https://en.wikipedia.org/wiki/Goidelic_languages