The 1835 tithes for the townland of Drumbulcan (standardized spelling) in the parish of Derryvullan list 2 Thomas Keys (senior & jr) who were almost certainly farmers:
http://www.irishgenealogyhub.com/fermanagh/tithe-applotment-books/parish-of-derryvullan.php
Griffiths Valuation (1862) lists 10 Keys households in Drumbulcan, nearly all farmers.
https://www.askaboutireland.ie/griffith-valuation/
One of the Thomas’s died in 1890 aged 80:
https://civilrecords.irishgenealogy.ie/churchrecords/images/deaths_returns/deaths_1890/06106/4742660.pdf
Another (a bachelor) died in 1879 aged 70:
https://civilrecords.irishgenealogy.ie/churchrecords/images/deaths_retu…
Isabella Keys (farmers widow) died in 1895:
https://civilrecords.irishgenealogy.ie/churchrecords/images/deaths_returns/deaths_1898/05840/4654014.pdf
Mary Keys (farmers widow) died in 1894:
https://civilrecords.irishgenealogy.ie/churchrecords/images/deaths_returns/deaths_1894/05967/4696002.pdf
The Church of Ireland baptism records contains several Keys births around the time you are interested in but none to a mother named Mabel. That said, some were illegible:
https://www.igp-web.com/IGPArchives/ire/fermanagh/churches/derryvullan-birth.htm
A Mable Johnston married Robert Swanston in 1812:
https://www.igp-web.com/IGPArchives/ire/fermanagh/churches/derryvullan-mar.htm
No sign of Thomas Keys death in 1850:
https://www.igp-web.com/IGPArchives/ire/fermanagh/churches/derryvullan-burials.htm
William Keys death in 1818 is listed, but without an age. Buried Lisnarrick.
6 Keys households in Drumbulcan in the 1901 census (30 folk in all) all engaged in farming and all Church of Ireland:
http://www.census.nationalarchives.ie/pages/1901/Fermanagh/Milltown/Drumbulcan/
Various Keys in the Freeholders List for Dumbulkin in 1796 (meaning they had enough land to qualify for a vote):
https://apps.proni.gov.uk/eCatNI_IE/ResultDetails.aspx
1766 Religious census lists Hugh, Thomas & William in Drumbulcan (all Protestant). Source PRONI names index.
Drumbulkin Keys marriage in 1865:
https://civilrecords.irishgenealogy.ie/churchrecords/images/marriage_re…