Chief celebrant Fr Michael Cusack described Adrian as "a virtuous man versus an individual who was consumed with evil".
"What happened to Adrian was a clear representation of the faces of good and evil. He was a man who so generously laid down his life for the community and his family. A man of virtue, a man of huge goodness, a loving husband and a loving son, a dedicated colleague. A very generous communityman."
Speaking to the Herald before the Mass, Fr Cusack said: "If we ever wonder about the battle between good and evil, all we have to do is look around us and see how evil creeps into the lives of anyone who allows it to sneak up upon them.
"There is nobody in the country who would not do anything to bring Adrian back. He is gone.
"So has evil had the final say? The reason evil does not have the final say is because the virtuous are rewarded with eternal life."
Fr Michael Cusack said the slain officer was "a perfect role model" who laid down his life for his community.
President Michael D Higgins, Taoiseach Enda Kenny and Garda Commissioner Martin Callinan were in attendance at the huge funeral at St Joseph's Redemptorists Church in Dundalk at 12.30pm this afternoon.
Hundreds of gardai marched in the funeral cortege as local businesses closed as a mark of respect to the officer who was gunned down by a gang during a raid on Lordship Credit Union in County Louth.
Cardinal Sean Brady led a large number of clergy at the funeral before burial took place at Lordship Cemetery.
Fr Cusack, Rector of the Redemptorists in Dundalk, had been requested to be chief celebrant by the murdered garda's widow Caroline because of his friendship with the couple over a number of years.
Each year, Fr Cusack celebrated a Mass in Dundalk Garda Station which was organised by Caroline, who is also a member of the gardai in Dundalk. The food served after the annual Mass was served by Adrian and Caroline, he said.
Fr Cusack said he, himself, was the son and grandson of gardai.
The priest spoke of his "great respect" for gardai because "they are the people who come face to face with evil".
Evil comes in the form of drug-dealing and para-militarism and murder and the refusal to give grieving families comfort by withholding the corpses of murder victims, he said.
"Adrian is a perfect role model for every Irish citizen, a loving husband, a doting father, a dedicated son, a loyal work colleague, a generous community man. May God grant him peace."