The Icelandic international had signed a two-year deal on a free transfer from Charlton.
Hreidarsson’s arrival put an end to Pompey’s interest in Titus Bramble.
Redknapp opted for the 32-year-old over the ex-Newcastle man because of his extra versatility.
Hreidarsson still had a year to run on his agreement with Charlton, but a clause in his contract said he could leave The Valley on a free after their relegation.
Pompey beat West Ham to his signature, despite the Hammers courting the player since the start of the year.
Redknapp told The News at the time: ‘Hermann’s a good signing. West Ham offered £2m for him in January but Charlton wouldn’t let him go because they were scrapping for their lives at the time.
‘But they went down and his contract situation meant we could do something with him.
‘He’s big, strong and a powerful defender.
‘Also, he has a great attitude and is a good character.
‘He is 32 but is as fit as a fiddle and has looked after himself brilliantly.
‘I’ve always liked him. I’ve tried to get him on a couple of occasions now.
‘I’m sure he is going to give us something extra and do well.’
Hreidarsson, who went on to play 120 games for the Blues, scoring eight goals, said at the time: ‘I was close to joining Portsmouth a few years ago. But the club is now a very different one with big ambitions.
‘I’m very happy to be part of a club that’s going to be pushing for a place in Europe next season.
‘I’ve always enjoyed the atmosphere at Fratton Park, but I would much rather have the fans on my side than against me. So I’m looking forward to playing in front of them.’
Redknapp was now pleased with the options he had at the back for the 2007-08 season, meaning a deal for Bramble was over.
‘I’m not sure about Titus now. Hermann came along,’ said Redknapp.
‘He can play left-back and gives us plenty of options.
‘The thing is, we have a few older players. Last year I dragged a few out to play who had injuries at times.
‘You don’t really want to do that in an ideal world. ‘It would be nice to be able to rotate the players.’