The reigning champions were left with a mountain to climb after entering the break 23-3 down and slipping even further behind thanks to Brian O'Driscoll's try just two minutes after the restart.
The fact that Wales showed exactly what they are capable of when they hit back with three tries of the own in the final 32 minutes made things even more frustrating for the Lions star, who admits improvements must be made ahead of Saturday's clash with France in Paris.
"In comparisons with the standards we have set as a team, that first half-hour was very disappointing and it cost us the game," said Roberts.
"It is one of the worst Wales performances I have ever been involved in.
The fact is, if we defended better in the first 20 minutes we would have won the match. But we were 20 points down before we touched the ball.
"It is hugely frustrating because we all know the structure is there for us to win rugby games but we made the wrong decisions on the ball against Ireland.
Despite the obvious disappointment of an eighth straight defeat, Roberts insists that the key to bouncing back in Round 2 is to stick together.
The man voted HSBC Player of the Tour when the Lions travelled to South Africa four years ago has dismissed suggestions that interim head coach Rob Howley may not have the full support of the squad and he is desperate to prove that Wales remain unified by producing a winning performance at the Stade de France.
"We are 100 per cent behind Rob Howley, without a doubt," added Roberts.
"Pundits are always going to say negative things but the important thing is we stay tight as a group. Saturday night hurt. I could see it on the faces of all the boys.
"We all know that where there are chances to score tries there are chances to win games and we know we can cause France problems on Saturday."