O'Connell picked up a string of man-of-the-match awards on European Cup and Test duty last season and was shortlisted for International Rugby Board player of the year as a result.

But his form dipped leading up to the RBS 6 Nations opener against Wales and it was not until the 43-13 rout of England that the hardman lock was back to his imperious best.

O'Callaghan, however, claims O'Connell should never have been questioned as his fellow Munsterman continues to be the spearhead of Ireland's pack.

'The criticism of Paul's form was complete rubbish,' said the Lions second row, who will be winning his 34th cap at Murrayfield on Saturday.

'I've been playing with Paul long enough to know it was typical - if the fella isn't casting bodies aside with big charging runs people expect him not to be doing stuff.

'But if you look at his stats during the RBS 6 Nations his work-rate has been incredible. Throughout he's done the unthanked work and it was very harsh to be down on him.

'I was delighted he put in such a huge performance against England to right the wrongs of what was being said.

'He's a class performer and he should never have been questioned. He's harsh on himself but that's the mark of the man.

'If something doesn't go well for him then he works harder than anyone else in the squad. He sets the standard.

'When he's a buddy of yours and you see this kind of stuff being written about him it's very upsetting.'

After producing a star turn against England, which landed him another man-of-the-match award, O'Connell has much to live up to against Scotland on Saturday.

But it is a challenge that confronts the whole Ireland team following a ruthless dismissal of the world champions that has put them within one win of their third Triple Crown in four years.

Adding spice to the contest is Scotland's desperation to atone for their self-inflicted 37-17 defeat by Italy and O'Callaghan knows they will enjoy being backed into a corner.

'Saturday is more about establishing the mental attitude of being professional and putting in a big performance,' he said.

'We must back up big performances back to back and that's what this week is about.'