Laura
It would seem from the submitted IGI entries, that someone has already been researching this McPherson (and/or the Baxter line). This looks like a submitted entry for Sarah Baxter's birth:
Sarah Baxter Christening: 22 NOV 1835 Ballyeaston Presbyterian Church, , Antrim, Ireland
Parents:Robert Baxter and Sarah Robinson
And siblings, also all submitted entries:
5. Robert Baxter Christening: 25 NOV 1827 Ballynure, Antrim, Ireland
1. James Baxter Birth: 18 JUL 1830 Of, Ballyeaston, Antrim, Ireland
2. Henderson Baxter Christening: 14 APR 1833 Ballyeaston Presbyterian Church, , Antrim, Ireland
4. Jane Baxter Christening: 10 APR 1838 Ballyeaston Presbyterian Church, , Antrim, Ireland
6. Thomas Baxter Christening: 06 JUL 1851 Carnmoney, Antrim, Ireland
I would imagine that your great grandmother Sarah McBride was called after Louisa's mother.
Monica