By the 15th century, with the accretion of Norse and English loanwords, the Scottish branch differed significantly enough from the Irish to warrant definition as a separate language. And nowadays these two languages, though have very much in common in dictionary and grammar, are different.
The alphabet of Irish and Scottish Gaelic is identical, consisting of 18 letters. Scottish Gaelic employs four cases of nouns: nominative, genitive, dative, and vocative (Irish uses three). Like Irish, the accent is ususally on the initial syllable.
Scottish Gaelic exists in two main dialects, Northern and Southern,
roughly geographically determined by a line up the Firth of Lorne to the
town of Ballachulish and then across to the Grampian Mountains, which it
follows. The Southern dialect is more akin to Irish
than is the Northern, and is more inflected. The main difference is the
change of the Indo-European é sound, which is eu
in Northern dialect and ia in Southern. Thus, the
word for "grass" is pronounced feur in Northern and fiar
in Southern. But most of linguists define much more varieties of
the language each having its peculiarities. In fact, Scottish Gaelic is
very endangered by permanent expansion of English and Scots. Nowadays only
on the Shetland Isles people can be found who speaks only Gaelic. On the
continent, everyone understands English, and slowly loses his native tongue.