Carolyn
I hope below will be of some assistance.
The surnames Bradley (not Broadley) and McGlinchey are still common in this area. Slevin as found on trip would not be associated. Civil registration in Ireland was not until 1865, it is hit and miss to find detail for earlier of births/marriages for earlier in particular Catholic records, Protestant churches vary in start date although some go back to the eighteenth century. The church records for Conwal and Leck which is the Catholic parish for Letterkenny are on microfilm which can be located on website, their baptism records start in 1853.
. https://registers.nli.ie/
Work is ongoing on uploading information from microfiche. In order to check on right path I scanned and found one of the family - Daniel’s birth in 1878, people were not always accurate with records as church has birth as December 1878 whereas civil was in January 1879. The church used latin for names hence the Francesca, below should link to this page-
https://registers.nli.ie/registers/vtls000631933#page/12/mode/1up
I can see that Family Search (https://www.familysearch.org/) while appears image is not currently available have Conwal & Leck transcriptions also.
I also found the civil record which you mentioned for the death of Fanny Slevin shoemaker’s wife in 1880, registration district Letterkenny, on original record it is difficult to make out age and feel the transcription of 50 may be incorrect, with present at death here Bridget. I used www.irishgenealogy.ie under civil records here. Other sites listed have more detail on the church records as this section is currently being updated.
As John had a trade it would have been an option for him to move from one area of Ireland to another. Word of mouth, suppliers of materials also advertisements would have assisted to find work in other areas. The link below can be used to show distribution of this surname, further south in Donegal and area around stretching to Longford have a heavy distribution. Below Irish Ancestors site shows breakdown of names by location.
https://www.johngrenham.com/
I have looked for articles on shoemakers and industrialisation meant that the shoemaker trade was in decline as factories mass produced. https://en.m.wikipedia.org/wiki/Shoemaking or https://divergentpathsstafford.wordpress.com/tag/irish-shoemakers/
It sounds great that another person has found a relationship link. In regards William Slevin and Hannah Nicols, the county of Longford has done great work in transcribing the church records hence in position to view this information but some other counties do not have anything or very little which can currently be found online, most records are from the time of civil registration in 1865.
Possibly you have already actioned but given the difficulty of finding other records and to broaden the checking process you could look at matching pool by uploading your Ancestry raw DNA data to a volunteer site called Genesis Gedmatch www.genesis.gedmatch.com ; You will get matches with people who tested with other companies and who have also uploaded their data to Genesis Gedmatch.
I also found Martha Slevin record death 19/11/1877 – widow of shoemaker. The district here is Churchill which is near Letterkenny, and for 1881 (13/4/81) found a Robert Slevin listed as a widower and death in same section of Churchill (Milltown) he is listed a farmer. There was a court case as son Patrick accused of killing. On Slevin search found on findmypast newspaper extract from Londonderrysentinal 21st May 1891 & Donegal Indepndent on 22nd May 1891 For free access Londonderry Sentinal check: https://www.britishnewspaperarchive.co.uk/
Below Irish Ancestors website provides excellent pointers for searching locations, names, parishes.:
https://www.johngrenham.com
For finding the catholic registers, the subsection:
https://www.johngrenham.com/places/rcmap_index.php
Donegal genealogy is a free site which has great local based information, including parish structures, there can be omissions as transcribed by volunteers, also note that some extracts may only have been copied for a limited time frame or from a specific source.
http://donegalgenealogy.com/
Hopefully other information will become visible. Good luck with DNA.
