I registered a business name. I submitted an application online.
and was sent a form with details already filled in. Once I had signed it and returned it with €20 my cert was issued after about 2 weeks IIRC.

I used the RBN number during the registration for my dot ie address which is the only thing I've used it for so far.

I think you can use it to open a business bank account as a sole trader. Tha account name would be 'Your Name' Trading as 'Your business name'