The marriage certificate won't tell you whether she remarried or when and where she died. There is no death of a Mary McMahon, other surname Paul, in the Scottish deaths indexes. If she died in Scotland, she should have been indexed under both those surnames as well as that of any subsequent husband, but it is possible that her first husband's surname was omitted if the informant did not know about her brief marriage to Maynard Paul.
The first thing to do is get the marriage certificate as Don suggests. Use the information in that to find her birth, and once you have her year of birth you can search the index for her death. She may not have died in Scotland, of course.
If she was born less than 100 years ago or died less than 50 years ago you cannot view the certificate online, so it might be worth getting a professional searcher who can transcribe all three certificates.
