Interestingly there is a bit of "Truth in Television" going on. Bagpipes are thought of as Irish and Scottish, but bagpipes were used widely across France, Italy, Spain, and the rest of the mainland Europe in the Middle Ages and onward (there's a whole section dedicated to the various styles of European bagpipes in Curt Sach's "The History of Musical Instruments"). In fact, Bagpipes date back at least as far as the ancient Greeks. Similarly, simple harps and primitive fiddles were also common in the Medieval and Renaissance eras.
A lot of Irish and Scottish music comes from an emulation of 16th and 17th century English Dance music, despite a rather strong dislike between the Irish, Scottish, Welsh, and English throughout history. That music in turn came from the Normans who conquered England in 1066 and brought their music and culture with them... The Normans were residents of northern France who, you guessed it, were originally what we would now call "Vikings"... hmm... COINCIDENCE!? I THINK NOT!!
https://www.youtube.com/watch?v=YkQzq5fOEK4
This music over time was preserved and transformed to fit on the instruments available, and a large variety of regional forms emerged that are still very extant to this day.
Now, that's not to say "Vikings invented Celtic music" or something like that. What likely happened was a general amalgam of existing secular music with the secular music of the waves of various invaders and the ever-present religious music of the all-powerful Church, which culturally dominated Western Europe, even having some sway to the East as well. Most of what is written down (at least in terms of notation) from the period is religious music, because it had such an important social and spiritual position that it was necessary to preserve it. Folk music and dance music was not really written down until later on.
What we can say is that Celtic music is pretty infectious, and it's actually a lot more ubiquitous than one might think- there are pockets of Celtic music all through France, and even parts of Spain and Canada. In fact, much of the Appalachian musical tradition here in the United States originates from Irish laborers (not to mention that English dance music had come over at the same time as well)- and that in turn fed into all modern American popular music genres. In a way, Celtic music has found its way into just about every corner of the musical landscape, perhaps subtly, but in a great number of ways.
Of course to say, that's not to say I don't agree- sometimes things are just a bit too "uh... wut" to call tasteful or dig through history to try to justify (*cough cough*). That being said, I genuinely believe the score to the first film at least is one of the better scores of the past decade (I honestly have yet to find one I found more interesting... everything is just so cliche, even questionable things sound refreshing).
