If you set up a company, you will be an employee of the company, and therefore have 2 employments. You will need to notify the Revenue of your new employment to have a tax certificate issued to you.
However, I think what you mean is that you will be self-employed. In this instance you will need to register for tax with a form TR1 - you should read the Revenue booklet "Starting in Business" available on the Revenue website.
In either of the above cases, I would suggest you seek professional advice.