Hi remington,
You can try this:
*) Before installing iRedMail, open iRedMail-x.y.z/conf/global, find below lines and change it to match your need (remote MYSQL server address and port). Then iRedMail will use remote MySQL server directly.
# MySQL server
export MYSQL_SERVER="${LOCAL_ADDRESS}"
export MYSQL_SERVER_PORT='3306'
*) Open iRedMail-x.y.z/conf/mysql, find below line and set correct MySQL root user name. default is 'root'. (NOTE: This parameter will be moved to file conf/global in iRedMail-0.8.4.)
export MYSQL_ROOT_USER='root'
*) During iRedMail installation, it will show you dialog to input password of MySQL root user, please type the correct password for your remote MySQL server.
Others keep the same. Let me know whether it doesn't work for you, so that i can fix it.