Quote Originally Posted by M. A. C. Newsome View Post
As to the difference between Irish and Scottish Gaelic, I asked the same question of a linguist once and was told that prior to 1700 the differences were minimal (effectively different dialects of the same language) but post-1700 the two began their own trajectories and are now considered different, though closely related, languages. So, rather like Spanish and Portuguese, native speakers of each could converse and understand a fair amount of what the other was trying to say.
Right. Earlier there was apparently more mutual intelligibility due to the use of a standardised literary form of Gaelic:

The Gaelic regions of Ireland, Man, and Scotland shared a common literary norm until the 17th century. The earliest writing is found in ogham inscriptions (ad300-500), a form of writing in which the letters are represented by groups of strokes and notches marked along the edge of stone monuments. Although the origins of manuscript writing in Irish are uncertain, the first extant text, a copy of the Psalms in the Latin script, probably dates from the 6th century. At that time, in any case, there began a great flourishing of literacy and learning in Celtic Ireland, and a substantial record has survived in Irish from the period. Lasting from approximately 600 to 900, it is known as the Old Irish period. The Viking raids of the 8th century disrupted the monastic order, which had guarded the literary norm, and about 900, with the beginning of the Middle Irish period, popular forms began to enter the language. The political turmoil that preceded the Anglo-Norman invasion of Ireland in 1169 may have led to a leveling in the forms of the spoken language. Whatever the case, by about 1200 what is known as the Early Modern Irish period had begun and a new literary norm (called Classical Modern Irish) had established itself. This standard was maintained throughout Ireland, Man, and Gaelic Scotland from the 13th to the 17th century.

In the 17th century, with the centralization of political power in London, the old Gaelic world was fragmented and its shared institutions destroyed. One consequence was the emergence of separate literary forms of Irish, Scottish Gaelic, and Manx, each reflecting local usage. In the case of Manx, continuity with the earlier literary conventions was completely lost.
(http://wiki.verbix.com/Languages/Celtic)