MBP 2015 with Iris Pro Only and External Monitor

Solution 1:

Graphics and Video support

Intel Iris Pro Graphics​ 2.2GHz - 256GB

Intel Iris Pro Graphics 2.5GHz - 512GB

Intel Iris Pro Graphics

AMD Radeon R9 M370X with 2GB GDDR5 memory and automatic graphics switching
Dual display and video mirroring: Simultaneously supports full native 
resolution on the built-in display and up to 3840 by 2160 pixels on up to two
external displays, both at millions of colors

Thunderbolt digital video output

Native Mini DisplayPort output
DVI, VGA, dual-link DVI and HDMI output supported using Mini DisplayPort adapters (sold separately)
Support for up to 5120-by-2880 resolution at 60Hz on a single external display (model with AMD Radeon R9 M370X only)

HDMI video output Support for 1080p resolution at up to 60Hz Support for 3840-by-2160 resolution at 30Hz Support for 4096-by-2160 resolution at 24Hz

retrieved from https://support.apple.com/kb/SP719?locale=en_US