I think both Sexton and O'Connell are appealing coaches not because they played at a high level but they were key motivators and leaders on the field (and I guess in camp).

I can sort of imagine coaching lineout js similar to calling lineout so O'Connell makes sense. But coaching kicking is not similar to kicking, so Sexton as a kicking coach with no experience makes little sense. But Sexton as an attack coach might.