5K external screen with 2015 15 inch retina MBP with Intel Iris Pro graphics
No, the mid-2015 15" Retina MacBook Pro is not powerful enough to drive a 5k-display.

You can see the specifications here:

Mid-2015 Retina MacBook Pro Technical Specifications

You're limited to driving up to a 3840x2160 pixels external display via the Thunderbolt/mini-DisplayPort port while using the built-in display. 

For the HDMI video output you can go up to 4096x2160, but only at 24 Hz. So still no 5k support.