Hi Julie,
I dont live near Killeshandra, but I will try and see what I can set out for you.
Do you know which church your ancestors belonged to Church of Ireland, Methodist or Presbyterian? I wonder what Concepta looked up..did she say? Possibly RootsIreland?? The following information is taken from irishgenealogy.ie telling you what is offline, look at that site and it will also tell you what is online. You may be able to contact them before you come.
WHAT CHURCH RECORDS ARE AVAILABLE FOR RESEARCH OFFLINE?
1. The National Library of Ireland has microfilm copies of almost all pre-1880 Roman Catholic parish registers on the island of Ireland. Access is free and print-outs of the records are allowed. See www.nli.ie.
2. The LDS Family History Library has microfilm copies of c. 40% of Irish Roman Catholic parish registers, some copies of National Library of Ireland films, others filmed by the LDS themselves. See https://familysearch.org/catalog-search. These films can be ordered via the Family History Centers attached to most Mormon temples.
3. Most local Roman Catholic parishes do not permit research on their original records. Callers will normally be referred to the heritage centres whose records are now almost all on www.rootsireland.ie. If necessary, contact details for local parishes can be found via www.catholicireland.net.
4. The Representative Church Body Library is the official archive of record for Church of Ireland records that survived the burning of the Public Record Office in 1922. A full list of its holdings is at http://ireland.anglican.org. All are freely searchable if they are not too fragile to be handled.
5. The Public Record Office of Northern Ireland (www.proni.gov.uk) has freely available microfilm copies of almost all surviving records of all denominations for areas now in Northern Ireland, as well as a good number for areas in the border counties of Donegal, Cavan, Leitrim, Monaghan and Louth. A full list is at http://www.proni.gov.uk/index/search_the_archives/online_indexes/church_of_ireland_index.htm.
6. Quaker records are very comprehensive back to the 17th century, with microfilm copies in NLI and PRONI. See www.quakers-in-ireland.ie.
7. Some Presbyterian records are only available locally or in the Presbytrerian Historical Society. See www.presbyterianhistoryireland.com.
Online do utilise all the records on https://genealogy.nationalarchives.ie, pay attention to the wills and the will calenders, again you should be able to order the wills before you come. On their census page you will find all the census including 1841, while it is not evident on the side when you select census years, that year is there...for free.
You will get Griffiths on http://www.askaboutireland.ie/griffith-valuation/ Look at Using the Valuation and Maps and it will guide you, sometimes, putting in less in the search option is best to get results, you are looking for townland names...take note of them
If Thomas was born circa 1828-1832, I wonder did his parents survive long enough to die within civil registration? Look for their deaths on irishgenealogy.ie and look for others with the same surnames, it may broaden out your research and get you back that one generation. Look also for death records through headstones look at https://historicgraves.com, billiongraves, findagrave, http://www.from-ireland.net/county-cavan-genealogy/,
I found this site relating to Killeshandra and they have a 'contact us' button, https://www.killeshandratourism.com/heritage-genealogy-killeshandra, I dont know how active this site is but it is worth a try. Also write to linda@ anglocelt.ie it is the local paper, there is no guarantee they will print it, but give them your best story with contact details and any photos will attract attention and may get you the details you seek.
You could take out a subscription with https://www.irishnewsarchive.com and look at the Co. Cavan papers, Anglo Celt, Freeman's Journal and even outside of that. The names you are seeking are not the most common, so it should be easy enough to trawl through them, make sure you do not auto subscribe by cancelling the subscriptions to any paid sites. They may have been involved in court actions, we were very litigious in years gone by.
I hope I have given you some new avenues to look up.
It is cold in Ireland in January, do bring the warm clothes, rain jackets and gum boots if you intend to trail around graveyards, the days are shorter, the library in Cavan town stays open late on Tuesdays and Thursdays.
Regards and best wishes Carmel O'Callaghan