How can you have an ATPL without the IR? I'm assuming you have completed all the ATPL exams and have used the IF experience required from operating elsewhere?
I believe you will find it difficult and expensive to complete an EASA IR on the 139. It's perhaps worth researching a course on a more common IR type like a 109/135/355 and timing it before your 139 PC where you can then get the IR add type?
LZ