Corrigan's shoulder muscle injury, sustained in training on Wednesday, has not responded sufficiently to treatment and he conceded his fitness battle when he sat out training today.
As Horan moves up from the bench to win his 14th cap, his place amongst the replacements goes to three times previously capped Simon Best of Ulster and Belfast Harlequins.
Reggie was nearly therebut not quite right and we did not want to take that risk said Irish team coach Eddie O'Sullivan, who added: if we had gambled and Reggie took another knock, it would put him out for a while.
Marcus Horan himself proved he had overcome the threat of a leg strain by taking a full part in today's 95 minutes training, in which Brian O'Driscoll also reported fully fit.
The only other players on the injured list are Gary Longwell, with an on-going calf strain and Anthony Foley, who has a knee injury.
Along with Reggie, they will be fine next week reported a relieved O'Sullivan as the build up to Ireland's more difficult Pool A games, against Argentina in Adelaide on Sunday week and against Australia in Melbourne the following Saturday evening, loom larger.
In that context, Ireland's all conquering coach - last weekend's win over Romania was N0. 16 in 18 consecutive Tests - says the selection of the back row is the most competitive within the squad.
He explained: Against Namibia we have gone without a pure open side flanker, which is unusual for us. The back row may not be as explosive as the one against Romania but it is certainly a more physical and heavier hitting unit. With the six players we have we are in the luxurious position of being able to pick horses for courses without weakening the back row. No matter what combinations we pick you have an International back row - and that's a great advantage for us.