Hi Ross
I don't think I am much help to you as I am only starting to research my Garretts.
All I can tell you is that my Margaret Garrett married William Howland at Andreas 29 Dec 1821. Their daughter Margaret married James Bell and they are my gg grandparents. It seems Margaret G snr then remarried to John Quirk at Andreas 19 Oct 1833. I have asked for a lookup for the MI for both Margaret Quirk and John Quirk but as yet haven't had a response. It is possible that Margaret [snr] was daughter to John Garrett and Ann Radcliffe who were married at Maughold 20 Feb 1787 but as yet I haven't proved this. There were various siblings to Margaret but I haven't researched them as yet.
If you find a link at anytime do please let me know. Happy hunting.
Viv