1. When a music lover can't tell the tunes apart, then the music is too fast.

2. At our English Country dances, we musicians are usually supplied with a metronome setting, but nobody knows where it comes from. So we practice it at the given speed. Then, at the dance, our leader listens to it, takes a few gliding steps to it, and then says "SLower." Sometimes she says it is good.

So it's not true that nobody knows what too fast is.

And by the way, this is not purely about Irish music. All kinds of music can be played too fast. Ragtime comes to mind.