With at least OpenWRT 15.05, the NRPE package appears to be unmaintained. We ~~could~~ should build the package manually, but before we do this, let's install an older version from our backups. For example:

$ ( cd ../backup/router/ && find . -name "*nrpe*" -o -name "check_*" | xargs tar -cf - ) | \
    ssh router "tar -C / -xvf -"

This should restore the NRPE binary, its configuration files and init scripts and all the check_* monitoring plugins. Did I mention that backups are important? :-)
With that, we're almost there:

 $ ldd /usr/sbin/nrpe
        libssl.so.1.0.0 => not found
        libcrypto.so.1.0.0 => not found
        libwrap.so.0 => not found
        libgcc_s.so.1 => /lib/libgcc_s.so.1 (0x77a64000)
        libc.so.0 => /lib/libc.so.0 (0x779f7000)
        ld-uClibc.so.0 => /lib/ld-uClibc.so.0 (0x77a88000)

Let's install the dependencies:

opkg install libopenssl libwrap

Add the nagios user:

echo 'nagios:x:50:' >> /etc/group
echo 'nagios:x:50:50:nagios:/var/run/nagios:/bin/false' >> /etc/passwd
echo 'nagios::16874:0:99999:7:::' >> /etc/shadow

Configure nrpe:

 $ grep ^[a-z] /etc/nrpe.cfg
 pid_file=/var/run/nrpe.pid
 server_port=5666
 server_address=192.168.0.1
 nrpe_user=nagios
 nrpe_group=nagios
 allowed_hosts=192.168.0.10,192.168.0.11
 dont_blame_nrpe=0
 debug=0
 command_timeout=60
 connection_timeout=300
 
 command[check_dummy]=/usr/libexec/nagios/check_dummy 0
 command[check_dns]=/usr/libexec/nagios/check_dns -H test.example.net -s localhost -w 0.1 -c 0.5
 command[check_entropy]=/root/bin/check_entropy.sh -w 1024 -c 512
 command[check_http]=/usr/libexec/nagios/check_http -H localhost -w 0.1 -c 0.5
 command[check_load]=/usr/libexec/nagios/check_load -w 4,3,2 -c 5,4,3
 command[check_ntp_time]=/usr/libexec/nagios/check_ntp_time -H 0.openwrt.pool.ntp.org -w 0.5 -c 1.0
 command[check_ssh]=/usr/libexec/nagios/check_ssh -4 router
 command[check_softwareupdate_opkg]=/root/bin/check_softwareupdate.sh opkg
 command[check_users]=/usr/libexec/nagios/check_users -w 3 -c 5

Let's try to start it, and enable it if it works:

 $ /etc/init.d/nrpe start
 $ ps | grep nrp[e]
 5320 nagios    2908 S    /usr/sbin/nrpe -c /etc/nrpe.cfg -d
 
 $ /etc/init.d/nrpe enable

And that's about it. Of course: since we're using an outdated NRPE version, we won't receive any (security) updates - so this setup should only be used in a trusted environment, i.e. not over the internet.