Holland and Barrett (http://www.hollandandbarrett.c...) stocks the cheapest Vegemite in Dublin that I've found, there are several stores in Dublin (see their website for details). The store in the St. Stephens Green Shopping Centre definitely stock it (next door to Argos on the first floor). Your mileage may vary with the other stores.

Nourish (http://www.nourishonline.ie/) stocks Vegemite, there are several stores in Dublin (see their website for details). The stores in the GPO Arcade and on Wicklow St. definitely stock it. Your mileage may vary with the other stores.

Vegemite can also be bought at Superquinn (http://www.superquinn.ie/) stores. The easiest to access from Dublin city centre would be the Blackrock store (or buy online!). Blackrock is worth the visit even if you don't want to buy Vegemite.

If you're looking for Vegemite in Ireland and you're not in Dublin then I'd probably start at the closest Superquinn.