Hi Jeannie
Have you read "Locating a parish" at:
http://registers.nli.ie/help
I too was stumped until I got my head around alternative spelling of surnames (e.g. Elwood can also be Aylwood). Have you tried wildcard searches using an * (e.g. **wood) ?
Also, another major problem in Co Roscommon Catholic Parish Records was the fact that the first name was recorded in Latin (and each priest had their own idea about which Latin name should apply to a Gaelic name). So I could not find "Bryan McDermott" anywhere on parish records until I realised he was recorded as "Barnabus" sometimes and "Bernardus" on other occasions. So you will need to check out:
https://dcms.lds.org/delivery/DeliveryManagerServlet?dps_pid=IE105460
Have you also tried all of these Roscommon resources...
https://www.johngrenham.com (to highlight the hotspots for any given surname in Co. Roscommon)
http://www.leitrim-roscommon.com/surname_intro.html (surname database, giving townland and parish)
https://familysearch.org/search (great for nailing down parishes... always my first point of call)
https://civilrecords.irishgenealogy.ie/churchrecords/civil-search.jsp
http://titheapplotmentbooks.nationalarchives.ie/pagestab/Roscommon/
http://www.igp-web.com/Roscommon/index.htm (The cemetery records are particularly useful).