'S i … Mura b' e i …
In the phrase 'S i a' Ghàidhlig … a feminine noun is used directly with the copula Is and the feminine pronoun i.
But in the last line we see Mura b' e i, rather than Mura b' i.
The meaning here is slightly different; MacGuaire is saying 'If it wasn't for her …'—the feminine noun is no longer being used directly with the copula, but with cha bu mhi (I wouldn't be) mi is directly used with cha.
Faodaidh an eadar-dhealachadh beag ach cudromach seo a bhith rud beag aimlisgeach.
'S i a' Ghàidhlig leam gach nì rim bheò.
The Gaelic is to me everything in my life.
Mura b' e i cha bu mhi.
If it wasn't for her/it I wouldn't be.
'S e Dòmhnall an tidsear.
Donald is the teacher.
'S i Màiri an tidsear.
Mary is the teacher.
'S e an giotàr an ionnsramaid as fheàrr.
The guitar is the best instrument.
'S i a' phìob an ionnsramaid as fheàrr.
The pipe is the best instrument.