There are differences in a lot of respects, and historical reasons for them.
Scotland was in an unusual position in the late 18th century, politically subservient to the British state but with substantial economic and cultural independence (the Hanoverians weren’t ripping us off like Thatcher did with the oil revenues). So it was strongly motivated to use national music as an assertion of identity, and it had a wealthy class that was willing to pay for it. So it developed a sizable music business that took the idiom and raw material of Scottish folk music and elite-ified it, creating complicated dance tunes and slow airs that were aimed at virtuoso display and often dedicated to rich patrons. (The raw material was mainly a mix of Lowland song and Highland pipe music, not much distinguished since this was a consciously national project). Irish landowners did that to a far lesser extent, and English ones hardly at all, since they had no pressing national cultural project to back and no reason not to patronize the pan-European genre of art music. There were English dance assemblies like the Scottish ones, documented in tunebooks, but they didn’t make a point of Englishness, and English tunes weren’t set off from the Scottish, French or Irish ones.
The result was that by 1800, Scotland had acquired a corpus of very sophisticated folk-derived instrumental music that nobody else in Western Europe could match. England and Ireland both had a musical culture divided into rather marginal and isolated musicians working as rural dancemasters, and an elite that played Italianate music in big houses. It took another three generations before the Irish started catching up with the Scots, and the English never did.
So the historical background to Scottish “trad” is that it wasn’t particularly trad at all, and came out of a very solid and active urban culture of dance and public performance. That’s never really gone away - Continental waltzes, quadrilles, polkas, sand dances and what-have-you American dance forms never supplanted the older stuff. And by the age of recording and broadcasting, the Irish church stomped on the free expression of indigenous dance at the same time as the British Royal Family decided to patronize the Scottish variety. (With mixed results in both instances).
The result is that it’s rare to find a Scottish tune session where at least some of the players haven’t played for social dancing. So the tunes get played in a way that suits that: much more definite accentuation than in ITM, with the beat underlined by harmony instruments (accordion for most of the 20th century). In the accordion and fiddle club scene the difference is even more marked, with snare or kit drums marking out the beat as well (this is only loosely associated with the session scene, but it does have a continuing influence) . And for social dance, the tune sets tend to be fairly fixed. (In the case of Scottish country dance, they are so rigidly prescribed that you really can’t do it without sheet music however good you are, the repertoire is too big and you can’t fake it - RSCDS players aren’t often found in sessions, but again they have an influence).
There are homogenizing tendencies. There’s a new market for highly virtuosic music which is primarily aimed at the concert stage and recordings. A lot of younger players mainly do this sort of thing. It can be hard to tell whether they’re coming from a Scottish or Irish (or French, or English, or Norwegian…) idiom unless they tell you.
