It’s going to be difficult to help you without a bit more information. Cassidy is a very common Fermanagh surname and Edward and Ellen are both common too. The 1901 census of Fermanagh has 15 Edward Cassidy and 16 Ellen. The name would have been equally common earlier in the century.
Not all parishes have records for the 1830s and that won’t help either.
Possibly DNA testing may be a way of matching with others who have additional information about where the family originate. Family Tree DNA reportedly has more people with Ulster roots than any other company. That obviously increases the chances of finding a match. You might want to try them or, if you have already tested, you can transfer your results to them for no fee.
The North of Ireland Family History Society is running an Ulster DNA project in conjunction with FTDNA and can offer testing kits at a reduced price. http://www.nifhs.org (Go to DNA project on the website).