It took the jury just over three hours of deliberations before they returned a unanimous 'guilty' verdict against Joseph Joyce.
As her husband was led from the dock and into custody by prison staff, Ellen Joyce dropped to her knees, started screaming and said: "My Joe didn't mean it. It wasn't meant to happen. God knows that. My children and me, we need him. We need Joe."
Mr McDonagh (18) sustained a fatal wound to his lower leg during a street battle outside the Enniskillen home of Joseph Joyce in April 2020.
In a trial that spanned three weeks, a jury of eight men and four women were shown CCTV footage which captured the fatal wound being inflicted on Mr McDonagh.
In the CCTV footage Joyce could be seen standing on the street outside his home.
Brandishing a slash-hook in one hand and plastic bottle filled with ammonia in the other, the footage captured Joyce engaging in what the Crown described as a "street battle" against brothers Gerard, John and Jimmy McDonagh.
The footage also captured Joyce administering the fatal blow to the back of Mr McDonagh's left leg.
Despite receiving medical treatment both at the scene and in the South West Acute Hospital, Mr McDonagh passed away on the morning of April 13, 2020.
Since his arrest following the death Mr McDonagh, Joyce made the case he was acting in self-defence and was protecting himself, his family and his Coolcullen Meadow home from an attack by the three brothers, who were armed with a knife, bottle and garden spade.
However, the jury rejected Joyce's claims of self-defence and found him guilty of murdering Mr McDonagh.
The father-of-three, with an address at Clon Elagh in Derry, was also found unanimously guilty of wounding Gerard McDonagh during the same incident, possessing two offensive weapons - namely the slash hook and bottle of ammonia - and a charge of causing an affray.
Before the jury returned their verdict, Joyce crossed himself in the dock of Dungannon Crown Court sitting in Belfast, and after the guilty verdicts, he was handed a life sentence.
Mr Justice Rooney thanked the 12 members of the jury for their service, then turned to Joyce and said: "The only sentence that I can give you is life, and so we'll come back and deal with the tariff at a later time.
"You will be notified when that will be, and your bail is revoked, obviously."
Joyce will be sentenced for the murder of Mr McDonagh at a later date.