You need to get a primary psychology qualification e.g. degree or conversion diploma, that is recognised by the PSI. Then the tough bit, get onto the Masters in Ed Psychology at UCD - full-time over 2 years. Ideally you shhould have experience in the area of learning disabilities, which I assume as a primary school teacher wont be an issue.
As you are working, I would suggest you look at distance learning for the primary psychology qualifcation. The OU has a degree and conversion diploma in psychology - both recognised by the British and Irish Psychological bodies. The Oscail psychology degree is still not recognised by PSI for some reason.