In the popular view, England was seen as the oppressor while the Celtic nations were seen as suffering from the same Anglo-Saxon terror. This is something you see in a lot of popular rebel songs and ballads, they talk about the Saxon foe. It's even in the Irish national anthem so there's always that romanticism about it.

As regards the IRA stance on it, the earlier edition of the Green Book (IRA volunteer handbook) frequently references England or the English where British might be more appropriate. This is something repeated in the newer 70s edition of the Green Book that the PIRA put out. There's frequent references to London and England instead of Britain/British/UK.

The PIRA stance is made pretty clear in their manifesto where it says "It stands with our Celtic brothers and the other subject nations of Europe" on pages 3-4 of the Green Book.

In practice (as you noted in your question), this meant that bombing targets in Britain were mostly confined to England. This sparing of Scotland and Wales did not however extend to Scottish or Welsh units on duty in Northern Ireland. They were a target just as much as the RUC or other regiments.

There's anecdotes from Welsh soldiers recalling graffiti in Welsh (generally mispelt) being sprayed outside their barracks with slogans like "Free Wales" or "Remember Tonypandy". I don't know if anything similar was done with Scottish regiments but Scotland has a much more complex history with the Troubles than Wales does (BBC's two part series on it is worth a look).

I'm not sure about Man or Cornwall but I can't recall mention of any attacks there either.

Sources:

IRA Green Book (50s and 70s version)

An Army of Tribes

The Dirty War