THe way maternity benefit works is that they count back 2 years from the start of maternity benefit to see if there were PRSI contributions paid then, i.e, if your wife had another baby in 2010 they would look to 2008 to see how many PRSI contributions she made, if she had the baby in 2011 they would look to see how many PRSI contributions she made in 2009. THink you have to have a total of 39 wks paid contributions to get maternity benefit.
Most questions are answered on the maternity benefit section of dep of family, otherwise give them a call they're very helpful.
Good luck with your decision