Marion said 'what notes to play or how long to play them - which is the point of writing music, isn't it? '

Yes, it's what notes to play, and how long to play them, and what emphasis to give them, and how smoothly to play them, and how loud, and what speed, and when to repeat and ...

With folk music, with well defined melody and simple rhythms (in general) it's fairly easy. The same way of writing music has to transmit much more complex ideas in other contexts, where the stress patterns are not so inherent in the melody.

People can clap in time to music because the musicians are giving the rhythm, and those cues can be readily picked up.

An interesting exercise - take 'Danny Boy', and re-write it as you suggested, with the 'Oh' on the fist beat. Play what you've written, with the stress pattern as we've discussed (strong on the furst beat of each bar). It's a different tune, although all the notes and note lengths are the same.

If you were a composer, and this new tune is what you actually want people to play, how would you let them know without using bar lines, or some equivalent way of indicating the stress pattern in the music? Yes, you could do it with stress marks all the way through, but that's not much different from using bar lines.

Bradypus

Bradypus