Red Hat does not natively support NRPE/Nagios with the default package manager. Luckily, DAG can save the day 🙂
Simply add the mirror spesified in the link above to your server and you get the ability to add the NRPE or other Nagios packages without too much hazzle.
In my case with a 64bit system, the way to get NRPE installed was as simple as this:
rpm -Uhv http://apt.sw.be/redhat/el5/en/x86_64/rpmforge/RPMS/rpmforge-release-0.3.6-1.el5.rf.x86_64.rpm
yum install nagios-nrpe