MBP 2015 with Iris Pro Only and External Monitor
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