Why is it "Irish" and "Gaelic" for "Irish Gaelic" and "Scottish Gaelic"?
When I started trying to learn Irish Gaelic, I was a bit surprised that they called it "Irish" instead of Gaelic, and was told that it's because there's more than one form of Gaelic. Now, I'm similarly baffled as to why they call Scottish Gaelic "Gaelic". Doesn't the same logic hold?
The confusing thing about Scottish Gaelic is that there is another language in Scotland called Scots. Scots is English’s closest living relative (or an extremely divergent dialect depending on who you ask) and it is not at all close to Scottish Gaelic. So Scottish people usually call Scottish Gaelic “Gaelic” to avoid the confusion. Irish and Manx, however, are usually referred to without stressing that they are Gaelic because there are no other languages they can be confused with.
I am not Irish nor Scottish, nor living on the Isles, so my view might not be entirely accurate, but this is how I understand it:
TLDR version: Irish sufficiently does the job (while Scottish on its own doesn’t) and Irish people (because of the English rule in the Irish history) want to emphasize unity of the Irish culture and language and distance themselves from anything in UK (like English language), thus Irish rather than Gaelic or Irish Gaelic (the last one leaves a possibility of there being other equally Irish languages, like eg. Irish English – not good).
Wall of text version:
First: Irish is the only native language of Ireland (considered a heritage language of basically every Irish person). No other major modern language developed in Ireland (except for Shelta, a minority language of Irish Travellers) – thus Irish without any other attribute sufficiently denotes the language. In Scotland, as ueck1 pointed out, always more languages have been spoken (Gaelic, Scots, Cumbric, Norn, Pictish)… of those all have been brought by foreign invaders in known history except for Pictish (which died out in early middle ages and we know very little about), so the term Scottish by itself doesn’t sufficiently denote any single language. In modern context it might at least refer to Scottish Gaelic and to Scots (the Germanic language closely related to English).
Second, the language originally evolved on the island of Ireland and was called Goídelc (hence modern autonyms of Goidelic languages today: Sc. Gàidhlig, Ir. Gaeilg, Gaelainn, Gaeilge, and Manx Gaelg, Gaeilck). Hence also the English word Gaelic (and a later academic Goidelic).
Third, since in Scotland always multiple languages were used, the word Gaelic (in the language itself: Goídelc, later Gaidhealg, and finally Gàidhlig) was always used to denote the language of Scottish Gaelic lords (and not Scottish Gaelic, since for most of the history the Scottish and Irish varieties were considered to be dialects of the same language, even if they differed a lot already in late middle ages). Because of that, sometimes, the word Irish might also have been used.
Fourth: the same name was mostly used for most of Irish history: both Gaelic or Irish in English (eg. Irish phrasebook of queen Elizabeth I uses the word Iryshe), Goídelc, then Gaoidhealg, Gaedhealg → Gaedhilg and Gaedhilge → Gaeilg, Gaelainn, Gaeilge in Irish itself, but the Goidelic langugae was the only native language there (hence the use of the word Irish in English).
Fifth: after the Famine, Irish language stopped being used on most of the Ireland, English replacing it. People started using the word Gaelic to distance themselves from the language (they felt Irish with Irish culture, not necessarily Gaelic, so they didn’t need to speak Gaelic to be Irish; Gaelic culture was the culture of poor peasants whose majority died during the famine, being Gaelic didn’t give you great prospects for life). Gaelic started being used a bit as a derogatory term.
Sixth: then, at the end of 19th century, Gaelic revival happened – a movement trying to revive the Irish language and Gaelic culture in whole Ireland – on one hand most organizations of that time still used the word Gaelic to refer to the native culture of Ireland and its language (and that’s still visible today in names like Gaelic revival, Gaelic League, Gaelic Journal, Gaelic Athletics Association, etc.), on the other it started being emphasized that the Goidelic language of Ireland is the language of the whole island of Ireland and the only native language there, thus – since the English name of the island is Ireland, the language is Irish and not Gaelic – you don’t need to feel Gael for the Irish language to be your language if you feel Irish. The term Irish, as I understand it, at that time felt more inclusive to people whose families haven’t known a word of the language for a few generations, thus more encouraging to learn it.
Also, Irish emphasized that Ireland had its own culture, distinct from the rest of United Kingdom. And that English is not a language of Ireland (the only one Irish language, not eg. Irish English and Irish Gaelic).
Seventh: after some time, somewhere in the 20th century, the name Irish almost completely took over the name Gaelic when referring to the Irish language in English, because of those political reasons. Today most people in Ireland were raised taught that their language is called Irish and not Gaelic or Irish Gaelic. And they like and keep that naming.
On the other hand, afaik there are people in Gaeltachts (the Irish-speaking areas), who still call their language simply Gaelic. And then, when speaking Irish, people use Gaeilge na hAlban, lit. ‘Gaelic, Irish of Scotland’ for Scottish Gaelic while in Scottish Gaelic Gàidhlig na h-Èireann, lit. ‘Gaelic of Ireland’ is used for Irish, as if they still were just different varieties of single Gaelic and no-one minds that usage. So generally the situation is a bit more complicated. ;-)
Best you can do is just respect the wish of the Irish to call their language simply Irish, and to respect the fact that there are more than one language of Scotland and call the language of Highlands Scottish Gaelic – that’s the most widely accepted nomenclature in English.
It wasn't always like this. When I was investigating what language you were allowed to use at Glasgow University, I found the only rule on the subject was written in 1481 - with no evidence it has been rescinded. It said disciplinary action would be taken against any student heard using Scotis. I had to ask my tutor what that meant, and what language you were meant to use instead. He said that it referred to the local vernacular. When I asked what that was, he said it was just at the point at which it was changing - Scots was taking over from Gaelic at that time in Glasgow, so it wasn't clear if it meant Gaelic or Scots. But he added that it did not really matter, as any peasant language was banned - you had to speak Latin.