Can you not look this up yourself? The airline web sites or your travel agent will be able to tell you what the excess baggage fees will be. Then go to the Irish post office website and see what they charge. You could also check out The Excess Baggage Co in Ireland prices here http://www.excessbaggage.ie/service3.htm
Note that sometimes adding baggage to your flights, it may be far cheaper to add one suitcase than two, so that two people with one suitcase extra each would be cheaper than one person adding two cases.
You'll have to do the maths yourself :-)
Edited: 2:47 am, May 08, 2013