The Munster lock played against the famous invitation side when Ireland were beaten 29-23 at Thomond Park five years ago – then helped the Barbarians beat a young England team 39-29 last summer.

The Barbarians – celebrating their 125th anniversary this year – tackle Ireland on May 28 before travelling to take on an England XV at Twickenham on May 31.

“There’s great excitement about the Ireland game all around the place and the Barbarians will be surprised how much Limerick will embrace them,” said O’Callaghan.

“Five years ago it felt like a home game for the BaaBaas. Quinny [Alan Quinlan] was playing against Ireland and the Munster supporters really bought into it. They wanted to see the Irish boys tested. We lost and there was a big cheer – it was confusing!

“For the Munster crowd it’s going to be great. They appreciate the traditions and values the Barbarians stand for and they know Ireland have to be ready for the World Cup. The crowd will want to see the Barbarians play well and see how Ireland handle the pressure.”

O’Callaghan won 94 Ireland caps between 2003 and 2011, taking part in three World Cup campaigns, but his call-up by the Barbarians was a special occasion.

“The moment you walk in you feel like an eight year-old going to your first game,” he said.

“You don’t really know the guys and in four days you’re going to be playing with each other. I loved it. The professional game measures everything and just to go and meet a group of lads and play the game you love is different.

“It makes you remember why you play and it made me love it even more. On the first day you’re thinking ‘how is this going to work?’ Then four days later it all comes together. It takes you back to why we do this in the first place.”

Playing with so little structure will make the Barbarians – coached by former Australia boss Robbie Deans – as hard to read as ever, and O’Callaghan is convinced Ireland’s head coach Joe Schmidt will learn more about his own squad as a result.

“For Ireland this is an opportunity to be tested against an opposition you can’t do any homework on,” said O’Callaghan.

“Joe Schmidt’s great on the details – but how can they defend if they don’t know what the opposition are going to be doing? There are going to be lots of guys who’ll have to deal with that and they all really want to put their hands up.

“This is also a chance for Ireland to learn something about the strength in depth of their squad. It’s not about 15 guys going to a World Cup. You need a scrap for every place on the plane.”