You will need several files:
/etc/lirc/hardware.conf – defines the hardware and points to the ir def file
/etc/lirc/lircd.conf – the default location for the ir def file. Associates the IR from each button with an action code
~/.lircrc – often and include file for the application specific files
~/.lirc/mythtv – the mythtv specific file. Associates the action code with an application and response
~/.mythtv/lircrc – typically a link to ~/.lirc/mythtv
You will also need to set the hdhomerun device to talk to the proper frontend. The details can be found at http://www.silicondust.com/hdhomerun/instructions/mythtv.
hdhomerun_config <device id> set /ir/target "<lircd ip addr>:<port> store"
You can find the hdhomerun_config tar at http://www.silicondust.com/downloads.
You can use irrecord to create your lircd.conf file.
irrecord -H udp -d 5000 "name of remote"
This command must be run with lirc NOT running. You can then copy the created file to /etc/lirc/lircd.conf.