Apologies if I'm misunderstanding your needs, but as you already have the IRR isn't this a straightforward calculation?

Assuming Principal is 100,000, Interest is 5% and a 20 year Period with annual payments (just to simplify) we get...

```
=PMT(0.05,20,100000,0,0)
```

.. giving the answer of 8,024.26.

Apologies if I'm missing the point completely!

Regards,

Brian.