Does mysql need to installed first or will iRedMail install for me ?
this paragraph indicates that it should not
WARNING: iRedMail is designed to be deployed on a FRESH server system, which means your server does NOT have mail related components installed, e.g. MySQL, OpenLDAP, Postfix, Dovecot, Amavisd, etc. Otherwise it may override your existing files/configurations althought it will backup files before modifing, and it may be not working as expected.