I will try and give you some directions, I live on the other side of Co. Cavan so I do not have the local knowledge. When they initially emigrated it would have been not long after the famine, so times would have been tough. When you say that they emigrated and not all at the same time, this indicates that you have these records and I am guessing that these records indicated that Owen returned to Ireland, do any of these records indicate a townland?
Regarding Martha Crawford, if you think that she was Scottish, you can use https://www.scotlandspeople.gov.uk they have an extensive collection of records, it is a subscription site. I had a quick look at RootsIreland (subscription site) but I could not find their marriage on any of the baptisms, but it may be because they are not transcribed.
I will list out the records that are available that should help you broaden your search particularly to take you before civil registration which is available on irishgenealogy.ie (free to search)
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 1851 for Tomregan but it is a partial census and only 2 families are listed on it, while it is not evident on the site 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.
You have indicated the parish of Tomregan & according to John Grenham the following church records are available for Tomregan:
Denomination Church Baptisms Marriages Burials Location Ref. Status
Roman Catholic Kildallen 1867 - 1881 1867 - 1881 PRONI MIC.1D/78 Open for public research
Roman Catholic Kildallen 1867 - 1899 1857 - 1899 CHGC Online transcript ($)
Roman Catholic Kildallen Apr 15 1867 - Dec 31 1880 Jan 1 1867 - Dec 9 1880 FHL (LDS) British Film Area 0979703 item 2
Methodist Ballyconnell 1880-1974 1880-1955 PRONI MIC.1E/53 Open for public research
Methodist Ballyconnell 1880-1955 CHGC Online transcript ($)
Church of Ireland Tomregan 1797-1875 1797-1845 1797-1875 Held by the Public Record Office in 1919.
Church of Ireland Tomregan 1797-1984 1802-1913 1805-1986 PRONI MIC.1/218 Open for public research
Note PRONI is in Belfast
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. It is very addictive, 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.
Look also for death records through headstones look at https://historicgraves.com, billiongraves, findagrave, http://www.from-ireland.net/county-cavan-genealogy/
You can also commission research from Genealogy Centre Contact Details
1st Floor, Johnston Central Library, Farnham St., Cavan
Tel: +353 (0) 49 4361094
I hope I have given you some new avenues to look up.
Regards Carmel O'Callaghan