Because you broke the import rules.
I quote from their site:
http://www.citizensinformation.ie/en/travel_and_recreation/travel_to_ireland/customs_regulations_for_travellers.html
"Goods
You are allowed to bring in to Ireland the goods shown in the table below, (subject to maximum limits) if you are travelling to Ireland from any country outside the EU (this includes the Canary Islands, the Channel Islands and Gibraltar).
The goods in question may have been bought either duty-free/tax-free or duty-paid/tax-paid outside the EU.
Allowances for travellers from outside EU
Goods
Maximum quantity allowed
Cigarettes
Cigarillos
Cigars
Tobacco
200 or
100 or
50 or
250 grammes
Spirits (whiskey, vodka, gin, etc.)
Intermediate Products (e.g. sherry, port, sparkling wine, etc.)
1 litre or
2 litres
Still wine
4 litres
Beer
16 litres
Other goods (e.g. gifts, souvenirs, perfume, clothing, etc.)
€430 per adult
€215 per child under 15 year"