You're right, the music is certainly a fundamental factor in a good show. My experience of watching close to 100 pyromusicals in Montreal is that the choice of music is harder than one would think.
In my opinion, it is important to get a good variation in rhythm and pace and so take the audience on an emotional journey. Bring them to points of high excitement, but then give them time to recover before bringing them back up. This type of formula is usually employed in the longer classical works such as symphonies etc. Music with a very constant level is very difficult to use successfully.
As for Celtic music, I'm sure I will be flamed for this, but I find that the melodic and rhythmic variation is probably not enough for anything longer than a few minutes. With the success of Riverdance, it has started to become something of a cliche in pyromusicals and the limited rhythmic variation is something of a handicap.
All, of course, in my humble opinion.
Paul. | 
