There's no such thing as 'duty free' purchases when travelling within the EU. There's only 'duty paid' (i.e. you pay whatever the local rate of tax is).
Ireland is one of the most expensive places in Europe to buy cigarettes. A post on this site states:
"A 20 pack of cigarettes in Ireland costs in excess of �6.50"
http://www.smokersclubinc.com/modules.php?name =News&file=article&sid=3664
However, that post was in October 2006. The Irish government increased the tax on cigarettes by 50 cents in December 2006. They then increased the tax by a further 30 cents in December 2007. So, even ignoring any price increases from the manufacturers, that statement must now be revised to read:
"A 20 pack of cigarettes in Ireland cost's in excess of �7.30".
If we take �7.50 as a price which is only marginally in excess of �7.30, that's roughly equivalent to �5.65 per packet. You'll probably find that the price of most brands is nearer to �6 per packet.
So you won't find any bargains (irrespective of where you shop)in Dublin. The residents of N. Ireland don't all rush across the border to buy cheaper cigarettes in the Republic. If anything, it may well be that citizens of the Irish Republic will head north, into the UK, to get cheaper prices.
Chris