This wiki is a community-maintained resource about everything there is to know about IPFire.
Check_MK is a Nagios or Icinga plugin similar to NRPE but with many improvements. One of the best improvements over NRPE is, that it supports inventorization. Another great features is, that it needs to open just one connection to a system to receive the state of all configured services and not one connection per service as NRPE.
check_mk is available in the stable tree since 2.15 Core 81.
You can use the check_mk-agent either via ssh or via xinetd.
As it makes no sense to invent the wheel a second time and the original documentation is good i recommend to read the original documentation on how to use check_mk via ssh.