Andrew Allen, 24, was shot dead though the window of his home in Links View Park at Lisfannon by gunmen from Republican Action Against Drugs.
PSNI and Gardai believe he may have been killed for travelling to Derry to see his two children. He had been banned from the city by the terror group for alleged drug dealing.
Mr Allen is being buried after Requiem Mass at Long Tower.
Parish priest Father Daniel McFaul said Mr Allen’s family were “devastated”.
“There’s no other way to describe it, they’re just completely and utterly devastated.
“This is a young man taken in the prime of his life.
“This morning my focus is on being with them and supporting them in their grief and getting them through the day.”
Mr Allen was one of 40 men forced out of Derry by republican vigilante groups last year.
Fr McFaul said the killing was “evil” and called for no retaliation.
“Andrew’s family spoke to me yesterday evening about his death and have asked me to plead for no retaliation,” Fr McFaul told mourners.
“To hear you Donna, his mother, a woman in the midst of grief, loss and pain, that no family should be put through what Andrew’s killers have put you through is an inspiration and an example to all of us”.