I sang this in Spiddal Galway Gaeltacht in 1957/8 when I was 8 years old so it obviously was not written by na cassidys. Some of the words are different .
An bhfaca tu mo Sheamuisin
Mo mhuirnean og mo bhuachaillin
An bhfaca tu mo Sheamuisin
Ag dul ar sios an mbothar

They have ‘mo stoirin’ for ‘mo mhuirnean’
and ‘is e gabhail sios an bothar’

I wonder also if they have put an extra verse in.
I’d love to have the original song.