Father Slevin's uncle was Father Patrick Slevin (1705-1788), born in Aghadarra, Dromore, County Tyrone, and later a pastor of this parish. Nicholas's father was Edward or Francis Slevin.
By May 12, 1783, Patrick, Edward and Francis Slevin "of Killbrack" (now Killybrack, 3 kilometers northeast of Omagh) occupied property in Dromore and obtained a joint lease on that date. They also leased a farm where first Penal Day chapel was built on the same site as today's parish church. (Presumably this is referring to the current-day St. Dympna’s Church of Dromore, as other works reference the chapel farm of this church being held by John Slevin in 1835, which fits with the information below.)
Edward is buried in the Old Dromore graveyard, his stone reading "This stone was erected by Edwd. Slevin of Kilbrack who depd. this life 26th November 1819, aged 89 years. Also the body of his wife Brigid McGinn who depd. this life Febry. the 6th, 1802, aged 72 years."
A nephew of Fr. Patrick Slevin finished ecclesiastical studies in the Irish College at Rome but fled there when Pope Pius VI was siezed by the French; he became a medical doctor in Paris. Another nephew, Surgeon John Slevin, was practicing in Trillick (County Tyrone, 5 kilometers south of Dromore) in the year 1817. A niece, Mary Slevin, married Patrick McGinn of Donacavey parish and was the mother of Rev. Dr. McGinn, Coadjutor Bishop of Derry. Another nephew was John Slevin, who held "The Chapel Farm" at Aghadarra, Dromore, which is still in the possession of his descendants to this day.
extracted from "A Letter of Nicholas Slevin, 1817" by Pádraig Ó Súilleabháin,
Clogher Record, Vol. 6, No. 3 (1968), Clogher Historical Society (Cumann Seanchais Chlochair),
edited by Rev. Joseph Duffy, St. Macartan's College, County Mongahan, Ireland.