Margaret b 1770 Wm Garrett/Isabel McBooy was alive in 1782 when Wm died.
Per summary in Manx Note Book
1782 A 47 GARRETT William dated 1 mar 1782;son john,paul,wm,patk;dau isabel,elinor,MARGT;wife isable exex;some ch ua;1788 thos quiggin h/o dau isobel

Isabel d in Braddan 1813 has a will
GARRETT, Isabella 1813 Bra E 1 0106415

I have more on this family but nothing further on Mgt.

What parish do you want MI?

DJD