Homeowners can typically reduce their energy usage by up to 20% by installing high efficiency boilers as a replacement for older lower efficiency models. There is now grant aid available through the SEAI Home Energy Savings scheme to help you improve your home using this measure.
Older conventional boilers can be wasteful of energy and costly to run because a significant portion of the heat is lost up the flue/chimney. The latest models of high efficiency boilers capture more of this heat, which would otherwise be lost.
For a full explanation of high efficiency boilers and how they benefit you, view the article from the SEAI here.