After reviewing the incident with the video ref, Barnes said: "He's come in from a distance at high speed and hit someone in the head. He's not in control, it's high danger. It's definitely foul play."
He became the fifth Irish player ever to be sent off, and the first in Six Nations rugby.
Wales Online headlined the incident 'Madness – Peter O'Mahony red card leaves Wales angry and fans in disbelief'.
Former Welsh international Jonathan Davies was among those condemning the act, saying: "That's a clear attack on the head. It's a red card, nothing else."
And former Irish forward Jamie Heaslip, a BBC commentator, said: "If we're looking after players, there's only one option the referee can give him."
Wales were returning to their beloved Principality Stadium, which had been used as a coronavirus hospital last year.
Cardiff is usually a vibrant city on big match day but the atmosphere was described as eerie during spectator-free Covid-19 times.
A superb first half from the great Irish playmaker Johnny Sexton saw the visitors lead 13 – 6 at the break.
Excellent finishing by wing Louis Rees-Zammit gave Wales the lead, but they had to withstand a powerful Ireland finish with a magnificent Justin Tipuric tackle on Garry Ringrose the defensive highlight.
Ireland were their own worst enemy with Billy Burns, who replaced the injured Sexton, missing touch with a final penalty, as he sought to gain maximum distance.
Sexton said: "We only have ourselves to blame.
"We spoke about discipline earlier in the week and that let us down at key times as well as some unforced errors."
But he also questioned some of the Welsh tackling which went unpunished, particularly a Johnny Williams tackle on Ringrose.
"We get red carded for a high tackle and they don't get penalised for one," he said.
Meanwhile the Telegraph said: "The knives were being sharpened for Pivac at the break but Wales just about scraped home…at least partly thanks to a bizarre finish (with) Billy Burns kicking the ball dead."
