You don't have to set up as a company if you don't want to, you can just register as a sole trader and make a return each year to Revenue. There is no problem doing this while being an employee also.
I'm no expert on it but there are revenue guides to self employment online and if you wanted to or had the time the local Enterprise boards usually run very good Start your own Business courses which are usually quite cheap and you get some great info.
Alternatively you could just meet with an Accountant who specialises in start up businesses, pay for an hour or so of their time and get the relevant info. Some offer first appointment free if they feel there may be future business in doing your accounts for you. I am a great believer though in asking more than one, no harm in having more than one point of view to make sure you are going the right direction.