I don't agree that you should be learning a tune and then adding the twiddly bits later. The ornamentation of a tune IS the tune. Play the ornaments right from the outset and you'll make them second nature in no time (as long as you also listen to lots of good players who play idiomatically).

C'mon, you know you're havering.

Tunes cross idioms or are made to serve different functions all the time. Almost all the oldest tunes in the Scottish repertoire started out as songs (and we can locate the words) - nobody sings the ornamentation that a piper or fiddler would use whan playing them as dance tunes. You've probably learned tunes that you play on the moothie from players of many other instruments - you don't do the semitone-wide glissandi that whistle players use routinely ("smears"), you don't have any way to get the sound a fiddler makes by ringing an adjacent string, and you can't do a slide that leaves 5 adjacent scale steps all ringing before you hit the melody note, as a harpist can. It's still the same tune, no matter if a harpist, moothie player or fiddler plays it, but the ornaments CANNOT be the same.

Breathnach's collection tries to allow for that by presenting a sort of abstract ornamentation system - identify the points in the tune that get ornamented, leaving out the instrument-specific details of how to do it. This is sensible, until you get to the point where the same tune is being used for different functions (e.g. a different kind of dance). Breathnach would probably write the reel and polka versions of the tune out as two different tunes, as his system doesn't have a way of marking an ornament "only do this for the reel version". An experienced player would just know to play the tune differently for the two different uses.

There are very, very few occasions in traditional music where an ornamentation pattern is genuinely idiosyncratic to a specific tune. In practice patterns of ornamentation are specific to particular styles - learning the style is about learning how to fit the pattern to any tune you come across. It doesn't have to be learned with the tune. One of the standard tunes in the Highland pipe repertoire is GS MacLennan's 3/4 march "The Kilworth Hills" - it's an adaptation of the Russian song "Stenka Razin". You can bet the Russian sailors GS learned it from weren't singing Highland pipe ornaments.

Example. The strathspey "Stirling Castle" (a "traditional Irish strathspey" according to the dumb fucks at TheSession). This is a minimally ornamented and rather boring take on it (Jeremy Button, fiddle):

https://www.youtube.com/watch?v=fLleYyDXSPE

This melodeon version (Curly Mackay) doesn't have much more ornamentation, but it's helluva dramatic thanks to the stretched dotted patterns:

http://www.raretunes.org/recordings/DeesideMelodies/

And this (John MacDougall, Cape Breton fiddle) makes it into a solo display piece, maybe not as danceable, but every detail of the tune is made to express something with unbelievably dense ornamentation:

https://www.youtube.com/watch?v=P2K7nvmnb_k

A version as a "Donegal Highland" - not much ornamentation, very much straighter rhythm than any of the others, but presumably it makes sense for that kind of dance:

https://www.youtube.com/watch?v=CcyUZYR5Mnw

That makes three versions of the same tune I could make an argument for as worth learning. But nobody in their right mind would try to learn MacDougall's version as an isolated artifact. What you'd learn would be two independent things: the tune itself, and how to play tunes his way. Having learned those, you are most of the way to knowing both how to play that tune in other styles, and how to apply MacDougall's style to other tunes.