Hi Hilts,
I will try and help you. Gettng back before civil registration which started for catholics in 1864 is difficult but not impossible. While you say there were baptised in Bailieborough, I was wondering do you have a townland? I would hope that they and their children born between 1835 - 1840 survived long enough to have some of their marriages and deaths registered after 1864. You can search for free on irishgenealogy.ie Also look at nationalarchives.ie under the tab 'Genealogy' you will find records relating to wills, land and census, though unfortunately the 1901 & 1911 Census are the only complete ones. I had a quick look at 1901 Census and you should consider those also living in Carnagarve & Skeagh as women that lived in those townlands most likely got married in Bailieborough. Prior to 1864, the church records are available for a fee on RootsIreland.
Philip Donnelly has documented his family very well, he runs a blog on http://donnellycanada.com/donnelly-families/ He mentions a Eliza Donnelly nee Coleman, do read his articles and see can you find any links.
The name Eliza & Bessy both derive from the name of Elizabeth, so it may be the same person.
Very often in the years you mentioned people married their neighbours, I had a quick look at the 1901 census and there is a McIntyre family in Skeagh, so I hope you can see how I approach the problem of identifying their townlands, it is not fool proof, but it is a start, the other issue is that McIntyre is also spelled McIntire & McInteer among other variations.
On the positive side the surnames Donnelly & McIntyre and not the most popular surnames in Co. Cavan unlike Clarke, Brady & O'Reilly.
I hope I have given you some direction, best wishes in your search and if you find something that you wish me to look at, please let me know.
Regards Carmel