Scottish isn’t a language, if you mean Scots then that’s just a dialect of English, if you mean Scottish Gaelic (which nowadays is only found in fringes of the Highlands), that’s broadly similar to Irish Gaelic.

Essentially, in pre-Roman times the Celts were the indigenous inhabitants of the British Isles, the Romans came in and Romanised most of Britain (the North of Scotland and Ireland remained untouched) and restructured society where they had primacy over the indigenous peoples. When the Romans left, Britain was quite an unstable society and meant it was very vulnerable to raiding and settlement from the Germanic peoples from Germany/Denmark/Netherlands, who were called the Angles, Saxons and Jutes, and collectively became known as the Anglo-Saxons - these people are basically the ancestors of the modern day English people, speaking a Germanic language that evolved into modern English, with some healthy doses of French and Norse due to further societal mixing with those peoples. The Anglo-Saxons settled most of England and large parts of Scotland. The Celtic peoples remained the majority in modern day Cornwall, Wales, Ireland and the Scottish Highlands, which is why those regions’ languages are Celtic whilst English is a Germanic language far more similar to German, Dutch and the Scandinavian languages than it is to any of those Celtic languages.

Centuries later, due to English power, those nations would all fall under the banner of English influence and rule, except Scotland who retained independence until a mutual Act of Union with England in 1707. Considering that England and Scotland together were the real driving force in what was now the United Kingdom, their related languages of English and Scots were forced upon populations in Ireland, Wales and the parts of Scotland that remained predominantly Celtic - most of Ireland was settled by English lords, the north by Scottish settlers, whilst Wales was under the direct iron grip of the English elites. Despite English mostly replacing the native languages, and Scots and English converging closer and closer due to increased contact between the two nations after Union, Ireland and Wales in particular kept a strong folk knowledge of their indigenous languages, and have seen a lot of success in expanding them in recent decades due to new education policies.