Hi Locke,
If you must go with either of these, I suggest you go with Eircom, while I find Eircom slow and there customer service bad, Vodafone is a nightmare, I signed up to the home broadband and phone, after two weeks I found out my landline could not take the broadband so I cancelled, well I taught, Vodafone sent me plenty of bills, which eventally they credited on there systems, but only after ring COMREG and complaining, I voiced my corcerns to Vodafone explaining to them I would never buy any of there products again, I would steer well clear of Vodafone, but if you decide to go with them if theres problems with the signal, Vodafone contact Eircom to fix the problem which takes longer than usual.
All the best
Chris