Market Infrastructure Resiliency Service.
This service is designed to be a standby system able to take over the principal functions of a (national) real time gross settlement system (RTGS) at very short notice, probably intra-day, in case of failure of a system or its primary back-up.
The Bank of England, for its CHAPS sterling RTGS service was the first subscriber to MIRS and how it uses it is discussed here. Not only is it a system dependent on totally different software, it uses hardware in multiple centres outside the jurisdiction of the RTGS system it is backing up.
SWIFT describe the service they provide here.
For treasurers, the main thing to note is that for payment systems using MIRS, the service is likely to be more resilient against failure.