Rosslea is in the RC parish of Clones. Unfortunately they have no baptism or marriage records before 1848, so if that’s where Patrick was born and married, then sadly there is unlikely to be any documentary record in the parish.
I looked in Griffiths Valuation for 1862 to see if there were any McElroy households in the Rosslea area then. There were 2 listed. One was Patrick McElroy in Lacky townland where he had a farm labourer’s cottage (Plot 1b). The other was Ellen (probably a widow) in Stralustrin. She had plot 1, an 18 acre farm. There were also 11 McIlroy which is an alternative spelling.
There was just 1 McAloon household, in Clontivrin. That was a Patrick, living in a labourer’s house (plot 1c).
You can see where they are in relation to Rosslea by using the maps on the Griffiths site.
http://www.askaboutireland.ie/griffith-valuation/index.xml?action=nameSearch
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 are running an Ulster DNA project and can offer FTDNA testing kits at a reduced price. http://www.nifhs.org (Go to DNA project on the website).