Did you ever work in a surgical specialty as a NP? You're an FNP so I'm guessing you didn't have any surgical training or rotations in your program...
The notes will all be pre-op, post-op and operative note style. Having been an IR RN will help. You know the basics that will be addressed in each note - which anticoags to hold and for how long, pre-op diagnosis, indication, labs and coags (pt/inr/ptt/plts), cxr, ekgs, NPO after MN, antibiotics on call, operative/anesthesia consents, moderate sedation medications, findings, specimens sent to lab/pathology, drains left in place, complications, disposition, EBL, etc.
Do you know what types of procedures they'll have you doing? In pain management I'm assuming you were at least doing joint/spinal injections. You'll need to become extremely comfortable using and interpreting flouro, US, CT and MRI. Knowing anatomy is essential and in general NPs are severely behind in that area. I'd get an anatomy book and absorb it. Read any and all literature you can on para/thoracentesis, all types of biopsies, LPs/myelograms, joint injections/aspirations - the basic and common procedures done by APPs in IR.
Good luck. IR is a tough specialty for NPs to break into.