In the UK, only CAA staff examiners can be IREs (examiners who can do the skill test for the initial issue of an IR), at least at the moment. There is a rumour going round this might change under EASA.
Us mere mortals can become CRE.IRR, Class Rating Examiner with IR Revalidation privileges. Was straight forward enough to get a few years ago (at a cost then of between 5 and 6 K and a three yearly renewal/revalidation cost of 1.5 to 2K), but with the above rumour in mind it would appear the CAA are not as keen to authorise people as they once were.
HTH
Gerard (CRE.IRR)