Hi
It could be that Mary Tully was married to someone else before she married John Madden - say for example she married when she was in her mid twenties, had a few children with that husband. When he died, she married John Madden, with whom she had the six children you know about.
So it's possible she could have had six children, in her forties. I know my great great grandmother had her last few children in her late forties, having started at the age of 22.
I assume the information you have on Mary Tully is from her marriage record? as you mention her father's name, but not her mother's.
I saw on the internet that there is a website called tullyfamily.com, but no idea if it would be useful to you or not, looks like you have to register and log on etc to use all the stuff on there.
Good luck!
Mags