Dear MckMac,
I wish I could help you find more direct ancestors. I have been researching my on family tree for over 40 years and I only got back to John O'Reilly born 1707, because a buried headstone was uncovered, however I am missing the details of his son, I do not even have his name.
Getting back before civil registrations which started in 1864 is difficult, most images are available for free on https://www.irishgenealogy.ie/en/ and you have to rely on substitutes. You may have exhausted the church records that are available on Rootsireland (fee paying) and any records available on http://www.genealogy.nationalarchives.ie (available for free). You can also contact Cavan Genealogy (fee paying) at
Genealogy Centre Contact Details
Cavan Genealogy
1st Floor, Johnston Central Library, Farnham St., Cavan
Tel: +353 (0) 49 4361094
Email: cavangenealogy@eircom.net
The substitutes for missing records are mainly land records and volunteers have transcribed some Registry of Deeds a link on how to search them can be found at https://www.irishgenealogynews.com/2018/01/registry-of-deeds-index-proje... If they are farmers search The Tithe applotments and Griffiths Valuations, while you will find names their dates are missing, but with some effort you can track the plots of land and who lived there and hopefully find a link to your family. For my own family research I have worked out the townlands on spread sheets that my family lived in, tracking the names and trying to prove when the names changed what the relationship was with the next name etc. I have placed these unrelated on my family tree and with some luck I have had matches with other family trees, which have either confirmed or disproved my research. The rental valuations in Ireland are available to view at the Valuations Office in Abbey Street. Family Search also have the images but to see them you have to
Also keep a look out for burial records, again volunteers have recorded some of the older graveyards, they can be found at various sties including https://historicgraves.com and findagrave. Families had traditional burial sites, sometimes travelling for miles to bury their dead so they are not necessarily buried in the most local graveyard or cemetery. For Shantemon, you should consider Castletara, Kill, Drung and Laragh. Not all are indexed yet and many of the older stones have deteriorated, but the volunteers have used many ways of reading them with some amazing results though many are still missing, so do have a look and see if there are any family names and townlands that seem familiar to you and keep a watch out for more records going online. I have also had some success with researching my family in Ireland beyond civil registration but it was through wills and records from America that aided me. If you had any priests in your family they are also well documented, if you give me names I will see if I can find anything here. You can also broaden out your tree by adding in siblings and family members as finding the marriage records of the younger family members hopefully can give you the parents names.
I have also used DNA, it will give you links and again you are trying to find names, which can be frustrating but it helps you change your focus and will help you find female relations that changed their names upon marriage.
I hope I have given you some more ideas to try and get your line back further.
Best wishes Carmel O'Callaghan