Tax credits can be deducted from your tax bill. Everyone receives personal tax credits. Depending on your circumstances you may be entitled to other tax credits.
A Tax Clearance Certificate ( cert ) is confirmation from the Revenue Commissioners ( Irish tax office ) that a person’s tax affairs are up to date at the date of issue of the Certificate.
If you want to be a freelancer or contractor, or you have been made redundant you need to look at your options
If you have a company car you will pay some benefit in kind - BIK.  Tax rules are that you pay PAYE on a cash equivalent calculated using Original Market Value and the mileage you drive for business.
There is a totally separate tax system for companies when compared to sole traders.
Most small businesses are organised as limited companies.  A limited company is straightforward to set up and it only takes 30 minutes to get it sorted out.
The Companies Office in Ireland are keeping the Register of Business Owners. 
When you are contracting or freelancing you could run your own limited company as it is the most effecient way to organise your affairs
The question of whether a person is employed or self-employed is a complex one.