There are several areas to watch out for. The Welsh style of play has significantly changed over the last few months with their backline in particular looking to move the ball into space no matter where they are on the pitch.
That style brings the likes of Shane Williams, Leigh Halfpenny and the impressive George North into the match frequently with vital early touches out in the wider channels. Factor in the roaming roles that back three have been given and you have a major area of focus for the Irish defence.
The catalyst for this change in mindset and style seems to have been the introduction of Rhys Priestland in place of the reliable but predictable Stephen Jones at out-half.
The back-row is also key. The speed, body positioning and technique of the impressive Sam Warburton and, to a lesser extent, Dan Lydiate and Toby Faletau means that the Welsh backs are comfortable attacking through the hands from anywhere because they believe their forwards will dominate the next breakdown.
If things go wrong they are also able to take it to the contact zone to reset with the likes of Jamie Roberts and Jonathan Davies in the centres, who can set a closer target for their loose forward trio.
Back-row battle - key to contest
Sat 8 Quarter final: Ireland v Wales, Westpac Stadium, Wellington
18:00 local, 05:00 GMT, 11:00 BDT
click here to watch Quarter final: Ireland v Wales rwc
Overall, the individual and sub-unit battles have to be won in order to get that collective victory and Ireland may have the edge because while Warburton is in superb form, Lydiate and Faletau are not quite hitting the same levels.
The combined form of the Stephen Ferris, Sean O'Brien, Jamie Heaslip trio, with their confrontational running lines and relentless workrate on both attack and defence, ranks them as one of the best backrow units on show at the tournament.
The new style of Welsh play could be their downfall because just as the standard 'position and possession' type of rugby can become very predictable, so can that constant attacking through the hands from anywhere approach. Putting pressure on Phillips and Priestland is the way to limit the width in the Welsh attack.
How do you do that? Ireland might use the following defensive ploy: first, send the third defender off the ruck hard at Priestland at first reciever, allowing him to go ahead of the defensive screen. The defenders inside and outside the "shooter" must keep their discipline and line integrity.
To limit width Ireland must press Phillips
That means Phillips, who takes a few lateral steps at times before he passes, cannot fiind his out-half with the pass and has to look to his physicality to try and penetrate around that second man off the ruck area.
It is then crucial that the two defenders beside the ruck concentrate on making a two man 'high choke' tackle on Phillips with one attacking the ball and the other chopping the legs. It is an effective defensive tool because at worst you have stopped the width to the Welsh attack and at best you have created a turnover situation.
One thing to remember is that if you over use this ploy then the opposition will start to use trailer runners off the scrum-half and negate the trap you have set - so you should only use this defensive play four or five times during a match.
What about Ireland's attack? They will be looking for excellence at the ruck this weekend as they know that their ball security and presentation was poor at times against the Italians, especially when they were in excellent attacking positions. To bring that high tempo game that played such a major part against Italy, they need to be precise and accurate in this area.
Ireland may change attacking plan
The attacking strategy could also change. Rather than play the 'wide to wide' pattern, Ireland may look to set closer targets early on in attack and not give the Welsh defence any chance of getting any impact hits or turnover opportunities out near the touchlines. Turnovers out there can be game changing moments.
By setting these closer targets in attack, they can use quick recycled ball using and one and two pass plays to keep the Welsh defence on its heels.
Done properly, that can create mismatches in the defensive screen as the Welsh defenders get sucked in to take up closer positions to the ruck.
I also expect the Irish set piece to carry on their good form from the Italian match. Wales will test them at the lineout with some early catch and drive mauls after seeing the Italians get some easy yards out of that. Gert Smal will have addressed that on the training pitch and his charges will be primed to negate the threat.
The Irish scrum will be looking to achieve the same objectives as last week - get parity and then step up the pressure on the Welsh scrum as the match progresses.
As I said at the start, I expect Ireland to win and take their place in the semi-final.
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