Intel Iris is a MESA graphics driver for Intel (i)GPUs using the Gallium stack. It was first merged in February 2019. The driver is meant to replace the i965 driver.
Iris supports Broadwell "Gen 8" graphics chips and newer. Older chips will still be supported by the i965 driver.
Iris is expected to be in usable shape with the release of MESA 19.2.
Irsi will eventually become the default driver for Intel (i)GPUs. Intel is aiming at making Irsi the default graphics driver by 2020.
Intel developers story is that Irsi will perform 10-15% better than the existing i965 driver.
Irsi got on-disk shader cache support in May 2019.
Trying it out
It's possible to use it on newer 5.x+ kernels with newer MESA versions by starting OpenGL applications by setting the environment variable