You don’t need to register a company in Ireland to conduct business as a sole-trader. Most people who work as contract programmers for example, earn money and pay taxes purely as a sole-trader and never incorporate as a limited company.
There are some advantages to forming a company but also several disadvantages such as overhead costs and legal requirements (see http://www.startups.ie/blog/index.php/sole-trader-vs-limited-company/) but generally for the amount of money you mention in your post, incorporation is probably a bit of overkill.
If you do decide to incorporate, it can be quite inexpensive (maybe a couple of hundred euro or less) using an accountant, or you can even submit to the CRO youself and save some money. Talk a look at http://www.mycompany.ie – I’ve used them and it worked very well for me.
Hope this helps.