Explanation/Reference:
HP supports IRF for multi-site extension. Because it supports connectivity over long-range 10GbE distances, IRF can be extended across dark fiber networks to bring switches dispersed among multiple sites into a single IRF virtual domain. This approach can be used to extend both LANs and SANs across disparate data centers.
Note 1: To create an IRF virtual device we have to connect members together through an IRF link. This link is created between 2 logical IRF ports, which are associated with physical ports in the member switches, that are going to form the IRF virtual device.

After a logical switch is created via IRF, member switches create an IRF virtual device, which acts as a single layer 2 switch, layer 3 switch (for all protocols) and shares single management IP address.
Note 2: Dark fiber is optical fiber infrastructure (cabling and repeaters) that is currently in place but is not being used.
Intelligent Resilient Framework (IRF) is a proprietary software virtualization technology developed by H3C (3Com). Its core idea is to connect multiple network devices through physical IRF ports and perform necessary configurations, and then these devices are virtualized into a distributed device.
References: HP Advanced Data Center Network Architectures, Key Requirements for Building Modern Virtualization Optimized Data Center Networks, Technical white paper, page 6
http://www.hp.com/hpinfo/newsroom/press_kits/2011/InteropLasVegas2011/DCRA.pdf
http://higaknowit.com/tag/irf/