Trying to find out about my great great great blind grandmother Mary Nolan's past

According to:

    http://www.census.nationalarchives.ie/pages/1901/Cavan/Killinagh/Cornah…

 my great great great blind grandmother Mary Nolan had to be born in Cavan county Ireland in the year 1831 or close to it, don't know her maiden name, she was married to my great great great grandfather Peter Nolan and he was born in Fermanagh county Ireland.  Well what I basically want to know is why and how Mary became blind, plus I contacted member mairead82102 that has a private family tree on ancestry.com (logged on over a year ago) that has a Mary Nolan that was born in Cavan county Ireland; Im curious to know if their is any family tree connection.

                            Thank you! :-)

                     Francis John Nolan