Is it possible to get IRB approval after the fact?
The human research ethics committee is in place to ensure that human research is undertaken in a way that safeguards the dignity, rights, health, safety and privacy of those involved. This assessment is difficult to make by the investigator because he or she has a vested interest in proceeding. Thus, an independent body is required.

In the institutions in which I've been involved, conducting human research without ethical review is, quite simply, research misconduct and subject to investigations and disciplinary actions. A priori ethical review is required. Regulations are in place that specifically disallow approval of research that has already been conducted.

I've been a member of institutional review boards and have chaired three -- one based in a hospital and two in research-intensive universities. I've been involved in these investigations in the past. As the original poster implies, most are found at the point in which the journal requires evidence from the IRB and the author quickly shoot an email to the office requesting one. In these investigations, it is often the case that there is a lack of training in the expectations and regulations of human research and offenders usually fall in three categories:


inexperienced researchers, including students (often poorly supervised) or practitioners with no research training
researchers trained "elsewhere", including those from countries with little to no requirements for ethical review
researchers from fields that don't usually involve human subjects


My worst case was a person from an electrical engineering background who collected data on body image for an app he was developing. During these sessions, the person gave advice that drove one participant to self harm. During the investigation, the offender did not think that ethical review was necessary because the questions were "innocuous (his term)".

As I said, in most cases the consequences are far less severe than this. For example, a common scenario is a young doctor who collects information from patient files, performs a few statistical tests and write up the results of this audit and sends the publication to a journal that then requests evidence of IRB approval. In cases like this, IRB approval is not provided and the doctor is placed under supervision.