Marion, I'm in England but brought up in the Irish tradition, so yes, I should have said that I was speaking from the Irish perspective. If a tune is written down and learned as a performance piece then it can obviously be played any way you or your teacher decide. However to play for the dancing of what I know as a hornpipe, you need the dotted rhythm.

An aside: Nic Jones plays the set dance 'Planxty Davis' on his album Penguin Eggs 'as it is written' in O'Neill's (The Dance Music of Ireland). I wish he'd heard somebody play it 'as it is danced to', before he recorded it, though I love the album.

Strathspeys I don't know so much about, as they're of Scottish origin, but I'll comment on what you said above, hope you don't mind:

What makes them different from hornpipes is: in hornpipes it is (almost) always the first note of the pair that is dotted and the second that has an extra quaver, whereas in strathspeys it varies freely which note in the pair is dotted.

What makes a tune sound like a strathspey to me is the presence of that rhythm where the second note of a pair is dotted, sounding like:
diddum__dum di diddum__dum.
And anything that makes you want to march is a march, whatever its time signature! I listened to 'Sidlaw hills' from your link above and it sounds like a strathspey but you could march to it too, so it's both.

Hope this makes sense!

Noreen