The effort expended in playing for over an hour with a man down took its toll in the final quarter against Wales, according to ex-Ireland hooker Shane Byrne.
Hefty favourites heading into the game, Ireland were instantly downgraded to clear underdogs on 14 minutes, when Peter O'Mahony was sent off for a reckless and unnecessary challenge, making contact with Welsh prop Tomas Francis' head as he went to clear out a ruck.
Byrne described it as "silly technique".
Ireland nonetheless rallied, completely dominating the second quarter and turning a 6-0 into a 13-6 lead at the break, the outstanding Tadhg Beirne scoring the critical try.
Ultimately, the effort couldn't be sustained. While the second half started reasonably well, errors started creeping into Ireland's play, allowing George North and Louis Rees-Zammit in for tries.
The hosts, coming off a dismal 2020 under new coach Wayne Pivac, remained jittery and Ireland were handed a late chance to steal it, inexplicably being handed possession with the clock nearing the red.
A fine break from Garry Ringrose took Ireland to the edge of the 22, Ireland subsequently winning a penalty but replacement out-half Billy Burns failed to find touch, putting the ball the wrong side of the try-line flag and ending the contest in a horrid fashion.
"Playing with 14 players for such a long time was always going to take its toll on the team," Byrne told Morning Ireland on RTÉ Radio 1 this morning.
"So many players stood up to be counted. When you end up numbers down, you want players to step up. Keenan, Henshaw, Van der Flier, Beirne, all of these players were absolutely outstanding.
"But it ends up taking its toll and it wasn't until Ireland started making mistakes that Wales got into it.
"Unfortunately, we'd Garry Ringrose's back-handed pass, a couple of penalties (given away) by Earls and Murray, Lowe stepping in, in defence (for North try).
"These small little errors, that were so absent for most of the game, in the last 20 minutes, started creeping into Ireland's performance."
It was Ireland's fifth loss to Wales in a game played under referee Wayne Barnes' direction. While there may have been debates over some calls, notably the extremely harsh penalty awarded against Tadhg Beirne to leave the visitors 21-13 behind, there is, as Byrne insists, little debate to be had about the major moment of the game - the O'Mahony sending off.
"It's just one of those things you cannot do anymore. You cannot make contact with the head. It was silly technique. There were other options available to him. It shouldn't have happened, end of story.
"The way the game is reffed now, there's absolutely no debate."
Next up is France, without a win in Dublin since 2011, but back in the groove under Fabien Galthie. Byrne admits it appears a stiff test from this remove but insists there's positives Ireland can take out of Cardiff.
"France weren't really challenged by Italy but from the little they showed, it looks like they're still in the form of 2020.
"There's no doubt there's a major challenge ahead but there was so many positives you could glean from Ireland's performance, the aggressiveness in defence, the way they manufactured go-forward ball.
"But they need to bring that all - with every player on the pitch hopefully - to the French game because that's the only way they're going to turn them over."
