well... tricky to define but I'll have a quick go. I'm sure somebody more eloquent will be along in a bit with a better reply.

Partly it is repertoire - some tunes are just traditionally defined as 'English' or 'Scottish' or whatever. Sometimes it's just fashion - down to what people in a particular place all liked to listen or dance to at a certain point in time. Especially if somebody then collected those tunes and published them as tunes typical of that region. If a tune has been published in a book called 'So & So's 101 best Scottish tunes' then people will be more likely to think it is Scottish... and there it all falls down because people in one region nick tunes from other regions all the time.

Partly it is the style - the speed, the rhythm, the smoothness or lumpiness of the individual notes, the choice of ornamentation... and so you could play an 'Irish' tune in an 'English' style, by altering some or all of those factors. As a very basic rule of thumb you could maybe say that English music tends to be lumpy and Irish music tends to be diddly and Scottish music tends to be somewhere in between (and I don't know much about Welsh music I'm afraid) but that is overly simplistic and possibly rather insulting to all of them :)

Partly it is the notes themselves - certain phrases or sequences of notes are typical of tunes from certain regions. Sometimes this will be down to traditional instrumentation - a lot of Scottish tunes for example were written on the pipes, and there are some runs or jumps between notes that are fall out easily under your fingers on pipes and some that are harder. (NOT that Scotland is the only country with bagpipes but don't lets get sidetracked by that one :) ).

Not sure if any of that helps to make a start?

Jess