well there you have it.
SC is to blame for not complaining, but the reason the SC folder is opening is because you have checked the box in post-cap form that says "launch external editor" but you haven't specified an "Ext. Image Editor" in that tab.
the proper thing would be for SC to pop up a message box telling you about this discrepancy.
so now you have a choice, you can uncheck that box saying "post capture commandline" and put the irfanview in "Ext. Image Editor", or you can uncheck the "Launch External Editor" checkbox in post capture dialog.
the advantage of doing the former is that you'll have a quick button in your main SC window at all times to open the image in irfanview.