I had a dispute with an employer a few years ago and met a rights commissioner.
I also had to go to the employment appeals tribunal on a different matter with the same employer.
Both experiences were OK and relatively informal.
Be sure to bring all your documentation, wage slips, time sheets,bank statements if paid through bank etc.
It is ideal if you are able to show (on paper) your loss of earnings etc. You can bring witnesses (e.g. fellow employees etc) but I did not think this was necessary in my case. I was awarded exactly what I lost in wages (1600 euros approx) and nothing more.
Best of luck.
PS Be aware that in most cases you will only be compensated for actual loss of wages that you can prove.