Generally speaking, any discrimination is totally forbidden.
People coming from outside the European Community are to ask for work and resident permits.
Immigrant Council of Ireland
As in most English-speaking countries, experience, abilities and references are more useful than diplomas, certificates and papers. Here, people like talking with each others and thus exchange a lot of oral information. You will probably be requested to give the phone numbers of your previous employers with a view to check if you are up to the job, and this is more likely to have them trust you than any certificate or graduation. As a consequence, improvements in working positions for serious and efficient people are supposed to be easier here than in the rest of Europe.
In fact, this is now far from reality: without a good English level, you can not find a good position. And to find a small job (in restaurants and pubs, etc.) nowadays becomes more and more difficult.
As for the attribution of a resident permit, a written note from a flatmate together with an ESB bill established at your name should be sufficient. More information: here
Form12A: Before starting any job in Ireland , you have to possess a special administrative form called the Form12A, used by the Tax Office to register you and follow up your tax payment process. Your employer is to fill it and will send it to the Tax Office. You can download it from the following link: http://www.revenue.ie/forms/form12a.pdf
In case you do not provide the Tax Office with a complete filled Form12A, they will class you under the "emergency tax system": the Tax Office will deduct up to 54% from your salary until they get all the information requested. Of course, you can recover the amount which should not have been deducted, simply by asking for it. However, you probably have better comply with the Tax Office requests right from the beginning.
P45: at each end of job (due to any reason whatsoever: end of contract, departure...), your employer has to provide you with a form called the "P45". In this form are written down all the salaries you have received in the course of your now ending job, together with the taxes which have been paid on these salaries. You will have to pass it on to your next employer. If you do not, the Tax Office may include you in the "emergency tax system", considering that you may be trying to avoid paying taxes in a regular way. If you do not find another job right away, you can go to the Tax Office with your P45 form and ask them for a tax back during your employment period. Moreover, it can be interesting to question the Tax Office over the exactitude of your tax payment. Once you get back to work again, inform the Tax Office of the identity of your new employer.
Summary: you need the PPS number to start working, the form 12A to register to the Tax Office for your first job, then the P45 for every new job when changing employers.