t6he igi (www.familysearch.org) gives some 6 possible Margaret Crows baptised on the Island within that time period - Crow is mostly a Northern name so most are around Ramsey which is where his mother came from (see www.manxnotebook.com/mxman/v12p002.htm
- so you have a number of possibilities - marrying in Douglas may mean her family had moved to Douglas - check 1851 census (tho I didn't see any obvious ones) or it could indicate that parental support wasn't looked for.

The transcribed reg for St Georges is available on mormon microfilm.