Four into three doesn't go and the performance of the back-row on Saturday has left Joe Schmidt with a most welcome headache.
The Welsh back-row outplayed Ireland in Cardiff and as a result there were plenty of calls for changes and even though Schmidt's hand was forced after Jamie Heaslip got injured, the balance of the trio against England worked very well.
If you look at CJ Stander, Sean O'Brien and Heaslip, they are all big, powerful ball carriers but Peter O'Mahony gives you something different. His shape in the lineout is so impressive and his spring allows him to soar through in the air.
Having him as an extra lineout option takes a massive amount of pressure off the likes of Rory Best, Devin Toner and Donnacha Ryan. And that's not just as a jumper; O'Mahony is a brilliant communicator at the set-piece.
It's the type of thing that you wouldn't really notice from watching from the stands, but O'Mahony is constantly talking to those around him. If you look at it next time, Ireland will have a huddle before they enter the lineout and a couple of options will be spoken about.
There will be a few key buzzwords used and even though Ryan was the one calling the shots on Saturday, you can be sure that O'Mahony's presence helped.
Luke Fitzgerald has backed former team-mate Johnny Sexton to start for the British & Irish Lions this summer after Ireland's 13-9 win over England in Dublin on Saturday.
Best's throwing looked very assured while England were under real pressure.
O'Mahony's 73rd-minute steal was the decisive moment. Everyone talks about Maro Itoje's athletic ability but O'Mahony is outstanding in that regard as well.
Most people can get lifted but having the agility and ability to shape your body whilst in the air is what makes a good lineout operator.
O'Mahony has a great understanding of the mechanics of the lineout. It's not easy to get your timing spot on and know where the space is going to open up but that's what he does so well.
I thought Owen Farrell might have went for the posts when they won the penalty after 73 minutes but they had the momentum with them and obviously felt that they could put the squeeze on Ireland.
But O'Mahony got up brilliantly to win the ball which was a massive boost and ultimately Ireland were able to see out the win. A huge moment from a big player.
I've said this since the start of the tournament that the lineout is a vital component and when the Irish coaches reflect on the last couple of months, they will rue the inaccuracies against Scotland and Wales.
There is a bit of a dog in O'Mahony and you need that from your six. He is as tough as nails and he brings that bit of anger and aggression that every coach wants.
That's the role, I played it myself, and of course there is a fine line. You want your six to be in the opponents faces and putting the pressure on and O'Mahony did that throughout. Of course the game has moved on and nowadays I don't think you can define a six as someone who just wants to be abrasive.
The way O'Mahony focuses on chopping low in the tackle is hugely effective. It's a technique that Dan Lydiate has perfected over the years.
For me, your six has to be a top class lineout operator but that doesn't mean that O'Mahony automatically gets picked for every game now.
You can't write Heaslip off for Ireland just yet. He will have been seething to have missed Saturday's game and since Stander has come into the team, you've seen his performances excel.
Competition for places drives the standards of everyone and Heaslip and co will understand the pressure that now exists in order to get selected. And that's how it should be.
When you underperform on the international stage, you should be feeling the heat. Second chances are few and far between but the guys who have proven themselves over a long period of time will get them.
Heaslip won't be dismissed but it will be very interesting to see what happens him in the summer. Not playing against England will have harmed his chances of getting picked by the Lions.
The front runners are Billy Vunipola and Taulupe Faletau. I don't think Warren Gatland will pick three number eights which makes you wonder if he will go on the Ireland tour to America and Japan.
Schmidt will use that tour as a chance to blood some of the younger guys but Heaslip will want to play because there are caps at stake. That said, we know what he can do and I don't think there is a huge amount of value in him touring.
We saw what happened at the last World Cup when five of our top players were missing against Argentina. When Japan 2019 rolls around, you will hope to have two or three quality players in every position to challenge for the trophy.
The quality of the back-row that we have is incredible with Tommy O'Donnell, Josh van der Flier, Dan Leavy, Jordi Murphy, Jack Conan and Jack O'Donoghue all pushing to be included.
Ireland had to withstand an inevitable spell of pressure but they did so incredibly well. Farrell had just cut the gap to seven points with a 53rd-minute penalty before they built several phases and looked threatening.
Another huge moment in the game was when Johnny Sexton, Robbie Henshaw and Sean O'Brien brilliantly forced a turnover with a perfectly executed choke tackle on James Haskell. It made me get out of my seat and the roar that that went up inside the stadium made everyone really believe that it was going to be our day.
The victory, as good as it was, doesn't make this Six Nations a success nor does it suddenly mean that Ireland are the best team in the world. The flip side of that is, it didn't make us the worst after the defeat in Cardiff either.
We do pessimism well in Ireland when we have a disappointing defeat but that is down to the standard that this team have set for themselves.
It's a long wait for our next competitive game but there is still a lot of rugby to be played this term. The make up of the team, particularly the back-row, will be fascinating come June and onto the November Series.
Finishing the Six Nations on a high by beating an outstanding England team won't be forgotten. It was a special day.
Eddie Jones said he could still be proud of his back-to-back title winners despite failing to break the world record for successive wins - and claimed the Aviva clash was akin to a World Cup final.