You have to remember that not all IR's are the same under JAR, but are more or less type specific.
For example if you fly a 737 for a living, your IR is only specific to that type, and unless you have done an IR renewal on a single pilot aeroplane the only instrument privileges you have when flying light single and twin aircraft are those of an IMC rating, which you automatically have because of your ATPL.
Hope this helps
Aerotowman