There may be an advantage in taxes and/or exchange rates to purchasing from a US-based travel agency like aa.co, expedia.com or AmEx.

I'm not familiar with Ireland specifically, but for travel in some countries, you can save on certain VAT-like taxes by purchasing your ticket from an overseas travel agent.

Also, if you book directly through Aer Lingus, you'll probably be charged in Euros. Foreign travel agents usually sell you the ticket in their local currency, typically at very favorable exchange rates (and without the 1-3% gouge fees most credit cards charge for foreign currency transactions).