Southern Ireland is the Republic - so isn't part of the UK and thus has it's own system, which I'm assuming isn't as high student fees? Although even Scotland has much cheaper fees and a different system than England, Northern Ireland and Wales.
I don't think that stevop21 was implying the Republic was still part of the UK, at least I hope hes not that stupid.
I think he was just making the point that UCAS points are used to calculate the CAO points equivilant for students in the UK applying for college courses in Ireland.