It won't be the entire south, and Welsh isn't widely spoken in the south. @Watchoutfortheslowaraf . Proportionally, South Wales is quite small, and the Welsh-speaking areas are mainly in the North West.
If you look at any Welsh pronunciation guide, E isn't pronounced 'Ah' in them.
As I pp, I have no objection to the name being pronounced the way the parents want; my objection is to the 'eira is Welsh for snow and is pronounced eye-ra'.
If they said 'my DD's name is Eira and it means snow' I'd be fine with it. Their child not mine.
Anyway. Miriam. How many syllables? Mirry-um, Mirr-yum or Mirr-yam?
Far easier (and nicer).