The voice service ( I presume you're talking about using the likes of Blueface? or do you just not need a phone at all?) will work fine on a 2mb line. It works fine on the NTL basic service 1mb line for Eur25 a month.
So when you say you want to eliminate line rental do you mean you hope to go voip over NTL?
with NTLs 1mb service, it will be same price as Eircoms 1mb service. ie, 25e a month. You'll only save if you get rid of Eircoms line rental thereby saving you the 25 a month rental.
NTLs 1mb line should be faster though as the contention rate is lower AFAIR.
Definitely the one to go for if you can get it...