If you are going to trade as sole trader, you must complete form TR1 – available from Revenue Commissioners.
(follow this link for the TR1 form).
For companies, you will complete the TR2 form which is also available from the link above.
COMPANIES REGISTRATION OFFICE (cro).
Limited companies must also register their business with the CRO.
This is also known as the company incorporation – ie the company is born !
You must complete an A1 form http://www.cro.ie/ena/cro_forms_tt.aspx and supple the legal documents Memorandum and Articles of Association.
The CRO website sets out in simple terms what is involved in setting up.
You may also buy a company off the shelf where all documents will be sourced and completed for you.