If you do the ATP you won't have to do an IR. The US ATP carries with it integral IR privileges. The test is basically an IR test with slightly tighter tolerances eg quarter scale deflection for the ILS.
I did it about 7 years ago. The only thing that has complicated it recently is the 'enhanced' 'security' procedures.
Once you jump through the 'security' hoops you'll need to do the ATP exam, a medical (class 1, I think. Can't remember) & the flight test.
If you're doing a type rating then it's often possible to include any additional ATP test items within the type rating check ride & get the ATP with the type.