Please ensure you use the same browser to login to CORE as you do to Revenue/ROS.
Click on the link below and follow instructions to register with ROS
https://www.ros.ie/helpcentre/help.html#/?a=gettingStarted/getting-started
Click on the link for instructions from the Revenue Site on how to create a sub user ROS Cert.
ROS Help Centre
https://www.ros.ie/helpcentre/help.html