Hi
The 1901 Census has been transcribed for 12 counties at the following site
http://www.leitrim-roscommon.com/1901census/census.shtml

Sligo County research library in Bridge Street Sligo has the original on microfilm
If your niece were to help you with this project,she can get the marriage details in the Civil records office Markievicz House Barrack Street Sligo.A copy of the certificate will cost €10.
I will do some more searching to see if I can find any more information

Regards
Rosaleen
http://underbenbulben.ie/home.aspx