Mr O’Neill, who worked for Dublin City Council, was on holidays in Majorca with his partner and three children when he was approached by assailants and shot a number of times in the back.
His children and partner were with him on the street outside a supermarket when he was shot.
The attack is reportedly linked to the ongoing feud between the Kinahan gang and the Hutch family.
However, Mr O’Neill was a completely innocent man with no connection to the feud in any way.
“Everyone liked him around here, there was no one who didn’t,” a neighbour of his home in Drimnagh, Dublin, told the Irish Examiner.
“Last time I saw him he was out playing football with his son. He was a big personality, very funny. We’re close around here. He was always out playing with his kids.
“There was nothing he wouldn’t do for you. It’s a big shock,” said the neighbour.
Mr O’Neill was in Spain with his family to celebrate his birthday.
A relative of Gerry Hutch, the alleged intended target of the shooting, was staying at the same apartment complex as Mr O’Neill.
The council worker was due home this weekend to take part in his annual neighbourhood street party.
“We do a thing for the kids every year it was to happen in the next few days,” said his neighbour.
His employer, Dublin City Council, described Mr O’Neill as a hard-working and dedicated employee.
“The management and staff of Dublin City Council are shocked to hear of the tragic death of Trevor O’Neill in Majorca last night. Trevor was a hard-working and dedicated employee of Dublin City Council,” said council chief executive Brendan Kenny.
Local Fianna Fáil councillor, Daithí de Róiste, said the entire community was shocked by the shooting.
“The community is in complete shock. It’s heartbreaking to see a young man, with three children, taken down in the prime of his life,” said Mr de Róiste.
“We’re averaging one murder a month. People need to be able to go on holiday and feel safe.
“We keep hearing about resources and we keep getting the same stale press releases and yet the murders continue. I’ll be bringing this up with the assistant Garda commissioner.”
Local Sinn Féin councillor Daithí Doolan also condemned the shooting. “This is a case of definite mistaken identity, wrong place, wrong time, wrong person,” he said.
“These criminals cannot be allowed to hold Irish citizens to ransom.”