What's the difference between Irish Gaelic and Scottish Gaelic?

What's the difference between Irish Gaelic and Scottish Gaelic?

Not much, as it turns out:

Scottish Gaelic is basically just an older, more conservative form of Irish Gaelic. 

The Scots or Scotti were originally a Celtic tribe living in northern Ireland. Between the 6th and 10th centuries A.D., some of them migrated to northern Britain then called Caledonia, now called Scotland. They took their north Irish Gaelic dialect along with them and it later evolved into Scots Gaelic. 

Scotland was not uninhabited when they came there. The Scots found other Celts already living there in the southwest (Strathclyde) who spoke a language very similar to Welsh called "Cumbrian." 

They also found a pre-Celtic people called "Picts" or "Caledoni" living in the Highlands. Unfortunately, their language is unknown to us and survives only in 14 inscriptions - some in ogham writing and some in Roman letters - which linguists have been unable to translate. 

Nevertheless, the Scots gradually merged with the Strathclyde Celts and Picts into a single Scottish nation. Their Gaelic language eventually replaced both the Cumbrian and Pictish languages but it's not exactly known how. 

Since the language of a colony always tends to be more conservative than what it is in the mother country, Gaelic didn't change as much in Scotland as it did in Ireland. So, ironically, Scots Gaelic is actually a little closer to the Gaelic spoken by such Old Irish heroes as Brendan the Navigator, John Erugena (or Duns Scotus) and Brian Boru than modern Irish Gaelic is. 

Linguists are divided as to whether Irish and Scots Gaelic are separate languages like Norwegian and Swedish or just strongly differentiated dialects of the same language like High German and Low German. 

Nevertheless, despite some differences, the two Gaelics still seem to be largely mutually intelligible between their speakers. One Irish scholar I talked to at a book fair once said that he could still understand 95% of Scottish Gaelic whenever he read it. 

There is still a third, little known type of Gaelic called Manx (as in Manx cats) which was once spoken on the Isle of Man between northern England and Ireland. Manx was a tad bit closer to Scottish Gaelic than Irish Gaelic and simpler than both. It's speakers were mostly Roman Catholic in religion. It died out not too long ago. The last speaker of Manx died in either 1957, 1962 or 1965 depending upon whose accounts you go by.