Energyquarter.ie is a useful website where the Energy Efficiency Upgrades section shows guidance from Joseph Little Architects on upgrading 3 types of existing houses.
The first one (shown above) is a cavity wall house built within the Building Regulations (after 1991), where cavity fill insulation is added and the attic is converted from a cold roof to a warm roof condition.
The second one is a single-leaf wall built from the 1950s onwards, with an upgrade including external wall insulation and new insulation over the ceiling.
The last one is a particular type of single-leaf wall, built between 1930-50 with concrete eaves, which is internally insulated showing both warm and cold roof conditions on different parts of the buildup.
Check all three in www.energyquarter.ie/pages/energy-efficiency-upgrades/