Question:
What is MIR in GSLB?

Answer:
MIR is Multiple IP response in GSLB feature.If you have configured the GSLB domain on the NetScaler appliance and if an ANY query is sent for the GSLB domain or the GSLB site domain, then the NetScaler appliance returns the IP address of the GSLB service that the appliance selects through the Load Balancing decision. If you have enabled the multiple IP response (MIR) option, the appliance sends the IP addresses of all GSLB services with the nest GSLB service IP as the first record and the remaining as additional records. By default this option is disabled on the NetScaler appliance. 

More details can be found in the following Citrix Documentation: http://docs.citrix.com/en-us/netscaler/10-1/ns-tmg-wrapper-10-con/netscaler-gslb-gen-wrapper-10-con/ns-gslb-protct-setup-against-fail-con.html