NSQ, 

Internal Rate of Return is calculated first.  This return does not change based on how much you then discount it.  It represents the rate of return an investor would realize if the discount rate is zero.   Another way to look at IRR is:  If they displayed a range of discount rates that went all the way up to 48.4% you would observe that the NPV would be zero.

The discount rate is used as a risk mitigation exercise for a prospective buyer.  It wouldn't be necessary to discount anything if we had a time machine and saw that at the end of the mine life that absolutely nothing went wrong, profits were predicted perfectly and inflation was zero.  With that guarantee you wouldn't need to discount the profits and you would be happy to pay the NPV0.  Your IRR and external rate of return would be identical.

They don't calculate the "external" rate of return.  They instead provide a discounted NPV.   If you want to calculate an external rate of return (post discounting) then in essence it is the IRR minus the discount rate.  

I hope this makes sense.