Consequently, Girvan Dempsey has been trust back into the limelight. He is the only proven international full-back in the party, divided into 17 forwards and 13 backs, and alternatives Paddy Wallace and Gordon D'Arcy have been left out of the squad.
Coach O'Sullivan said: "It (Murphy's injury) did have an effect. He is a player that can cover a number of positions. David (Humphreys) is a possibility at full back. So is Denis Hickie. So is Brian O'Driscoll. So is Jonny Bell.
"Some of the players who haven't made it in are good enough to play in Rugby World Cup which is sad for them, but I think we have the 30 best players available to us in that squad".
"Not to beat it to death, but we said the back five was the area of most contention and competition. No matter what names were left out of there, you could talk about it all day.
"We are bringing an extra back row player. We have such a wealth of talent there and it does give us lots of options in the pack which is where the stall will be set out.
"We picked a Rugby World Cup squad two years ago and it was very different to what it is today. That is the nature of the game. Nothing stands still. Players improve and they still don't make it. Other people pass them out. It's a very fluid and changing situation - rugby. It's the here and now that counts," he said.
Keith Wood's return to fitness and the reduction in Frankie Sheahan's suspension has solved a potentially disastrous dearth of quality in the hooking position.
"I think it's great that Frankie is back in the game. It's great that he came back on the bounce. He did very well on Friday night. It probably astounded a lot of people that he lasted the whole game. It was a fairly high-octane encounter in Donnybrook. He has shown a lot of resilience".
In the final analysis, Simon Best held off the challenge of Justin Fitzpatrick and the injury-prone Emmet Byrne.
"In the front row, the prop position, Justin Fitzpatrick and Emmet Byrne losing out was tough on both of those, admitted O'Sullivan.
"I think I can say, hand on my heart, that every sector was difficult, not for the first two or three positions, but the final position".
There are still a few injury worries that need to be rectified in the camp. "There are a few players in that squad that are currently injured. We have Shane Horgan, John Kelly and John Hayes and they will be given every opportunity to prove their fitness.
"Shane is due to be back playing on the week before we go into final camp. John Kelly - we don't know. It's just a rib cartilage, which depends on his healing power. There's nothing we can do with that. He has made a good recovery in the last week. John Hayes may take a little bit longer. Jonny Bell had a bit of trouble with an achilles tendon and we will review that next week".
Ulster's Neil Doak is the only uncapped player in the squad: Neil (Doak) was ahead of Brian (O'Meara), in the pecking order, earlier in the year and sustained a shoulder injury. He worked very, very hard to come back for it and just made it at the bell." explained O'Sullivan.
O'Sullivan's sensitivity and loyalty to his players shone through in his admission that he had only phoned the players that didn't make the final cut.
"I made calls this morning to nine players who had every right to feel they could be in the squad. It was a very unpleasant task. Certainly, I can safely say I had no breakfast and I haven't had one yet.
"It was one of the most difficult things I have ever had to do. It's bad enough to leave a player out of a test game, but to leave a player out of a World Cup is harrowing and there were some very disappointed men this morning.
"To their credit, they all took it on the chin. It shows the strength of character of the guys we have in Irish rugby. They are true professionals.
"I didn't ring the players who were in. I really think that when you are doing something like this, it's easy to give the good news. To tell players they are in, I don't think they need to be told. It's the guys that you don't put in that are the guys you have to talk to. I haven't rang anyone who is in the squad.
"Looking forward, it's great to have the squad down on paper. I look at it and I am very comfortable with it. I am happy with the squad. This is what we have going forward. I am very happy to go to Rugby World Cup with this squad.
"I think it was a very productive three weeks, apart from the injuries to Rob (Henderson) and Geordan.
"The pressure of whether they (the players) are in or out of the squad comes off them and there is a relief in that, but this can't be the top of the mountain, getting in the World Cup squad. This is base camp and the top of the mountain is a long way in front of us. That is what the players need to realise,"said O'Sullivan.