I bumped into Ieuan Evans, of all people, walking down Lansdowne Road this week. Never toured with Ieuan but we’ve crossed paths at British and Irish Lions functions. And I’ve seen the videos of his tries for Wales.

Evans was over to chair a Lions meeting so I asked about David Wallace’s ascent to the board in place of Tom Grace.

It is on weekend’s like these, when the pitches are firmed up, that you want Wally taking no prisoners on the openside flank.

Ireland were blessed that Seánie O’Brien and Jack Conan hit their peaks in quick succession after Wallace. But Munster people won’t forget his impact over 14 seasons until a horrendous knee injury sustained in contact with Manu Tuilagi during a World Cup warm-up in 2011.

Despite turning 50 this summer, I hear Wallace is lifting heavy in the gym. He still has the cut of a professional backrower. That is some going.

European semi-finals are the sort of days that turned men like David Wallace into legends of Irish rugby.

Two vastly different situations will unfold in Dublin and Belfast on Saturday. Hopefully we get the same outcome to wash away the doom and gloom from earlier this season, when Ireland lost to South Africa and to France in Paris, by two provinces reaching their respective European finals in Bilbao.

The test for Leo Cullen’s Leinster is mental as much as it is their concerns at scrum time. Andrew Porter has returned but no squad can prepare for injuries to Jack Boyle, Paddy McCarthy and Alex Usanov, while Ed Byrne is cup-tied.

For Leinster, getting past Toulon is as much psychological as it is about their issues at prop, with Porter short on match fitness and Tadhg Furlong unavailable.

Leinster's Jamie Osborne celebrates scoring a try with Jordie Barrett during last season's URC semi-final against Glasgow Warriors. Photograph: Dan Sheridan/InphoLeinster’s Jamie Osborne celebrates scoring a try with Jordie Barrett during last season’s URC semi-final against Glasgow Warriors. Photograph: Dan Sheridan/Inpho

Leinster were built to conquer Europe last season. All the pieces were in place. RG Snyman is injured and Jordie Barrett is away home, tearing up trees at the Hurricanes.

It must still haunt them that Porter, Barrett and Conan were benched for the semi-final loss to Northampton.

There are plenty of learnings – as we like to say in rugby – from that day at the Aviva Stadium. And plenty of cost. Caelan Doris missed a chance to captain the Lions in Australia as he damaged his shoulder trying to lead the turnaround of a 27-15 deficit.

That’s probably the biggest lesson of all. When you are chasing down 12 points in a home semi-final, all the natural advantages start to work against you. The place goes quiet. Even the tiniest error is exacerbated by a collective groan from the crowd. The nervous energy spreads like wild fire. No team wants to be playing against two enemies: the clock and the opposition.

Leinster know they have to hit the ground running and not let up. That’s how they always want to play, at a high clip.

The big decisions have already been made. At outhalf, Harry Byrne is charged with ending an eight-year gap to their last European trophy. Sam Prendergast makes the reserves ahead of Ciarán Frawley, having recently missed out on the matchday squad. Ireland’s number 10 at the start of the Six Nations, Sam can end up having an influence in Leinster’s biggest game of the year.

Really, Leinster’s campaign only got going when the Triple Crown celebrations died down. But this is their cross to bear. They won’t use it as an excuse. Sure, they are back in a home semi-final despite an enormously challenging season after 14 players were involved in a Lions series that ran into preseason.

This included Jamie Osborne and Thomas Clarkson, two fringe players until their respective breakthroughs. With Tadhg out, Clarkson starts at tighthead while Cullen has gone back to Robbie Henshaw at 12 ahead of Osborne.

To stitch another star on the jersey, a similar showing is needed from what Ireland produced at Twickenham in February. Many of the same players are involved.

There are X-factor athletes in both provincial squads. Leinster do not have Barrett any more but in recent weeks Rieko Ioane is looking like the All Blacks star they hoped to see on firmer ground. Ioane on the left wing ahead of James Lowe is another big call.

Nick Timoney's destructive performances for Ulster have been a revelation. Photograph: Charles McQuillan/Getty ImagesNick Timoney’s destructive performances for Ulster have been a revelation. Photograph: Charles McQuillan/Getty Images

We’ve seen Ulster progress this season under Richie Murphy but that won’t be remembered if they lose a Challenge Cup semi-final at home to Exeter and fail to make the URC playoffs.

Last weekend’s heavy defeat to Munster, when Murphy sent a second-string squad to Limerick to avoid further injuries, made it clear that they aim to qualify for the Champions Cup by winning the Challenge Cup.

There is an element of risk to this approach. They have slipped to eighth in the URC with Connacht three points behind them and all set to make a late charge for the last playoff spot.

But the selection last weekend made sense as both Ulster players and fans want to end a 20-year trophy drought.

They have a real chance of making the final. We saw how Exeter knocked Munster out of Europe with some ferocious defending, not letting my former team over the gainline at Sandy Park before the class of Henry Slade came to the fore.

Over to Stu McCloskey and a fit-again Jude Postlethwaite to shut down Slade in midfield and deny space for Immanuel Feyi-Waboso to run riot.

Ulster can get the job done in Belfast with the return of Nick Timoney and Tom O’Toole both significant. O’Toole has been invaluable to the Ireland squad, by playing loose and tighthead, while Timoney’s destructive performances have been a revelation.

Not to me, mind. Josh van der Flier is the established openside for Ireland yet he needed to maintain World Player of the Year standards to keep Nick at bay for so long.

I’m excited to see Bryn Ward’s impact off the Ulster bench. Only 21, I spoke to Bryn on the BBC Rugby Social podcast recently with his brother Zac. He’s a great character. Fearless. Loves the physical stuff.

Like Wally of old.