Irish Water, the state forestry company Coillte, and the HSE between them account for one third of all the energy consumed in the Irish public sector.
The information is contained in the latest annual report on Public Sector Energy Efficiency Performance published by the Sustainable Energy Authority of Ireland.
The report analyses the progress being made on reducing energy consumption by 354 public bodies including all Government Departments and agencies, as well as also almost 2,700 schools.
It finds that a total of €1.3 billion in energy costs has been saved by improvements in energy efficiency across the public sector since 2009.
The effect of that has been to prevent the emissions of 4.6 billion tonnes of carbon dioxide into the atmosphere.
Taking all organisations and schools together the overall improvement in energy efficiency to date has been 27% and the SEAI says the public sector is now on course to achieve its 33 energy efficiency target by the end of 2020.
The efficiency gains have been achieved through the implementation of thousands of diverse projects ranging from structured energy management, building and facility upgrades, retrofits, changes in transportation, better energy procurement, and through behavioural change in organisations.
Last year alone a total of 4,036 energy efficiency projects were reported to the SEAI by organisations and schools covered by the report.
For instance two energy-saving projects were completed at the Central Statistics Office headquarters at Mahon in Cork last year.
There, older gas heating boilers were replaced with four cascade boilers as well as area zoning controls for greater control over the heating system.
In addition, existing fluorescent light fixtures were replaced with LED lights incorporating occupancy-based controls and daylight harvesting.
So far the project has reduced gas consumption by 171,100 kWh, electricity consumption by 125,700 kWh, and shaved €25,000 off CSO's energy bills.
Maynooth University improved its energy efficiency by 34% since 2009 through the implementation of energy efficient practices, plant upgrades, LED light replacements and enhanced building system design.
The university also provides a series of electric vehicle charging points to promote electric vehicle use among staff and students.
Minister for Communications, Climate Action and the Environment Richard Bruton said: "The increase of a further three percentage points in energy efficiency across the board is to be welcomed and will help ensure we reach our 33% energy efficiency target by 2020.
"However, the variation across the board is cause for concern - 162 public bodies delivered more than 30% energy efficiency over the period but 49 either deteriorated or improved by less than 10%.
"The Climate Action Plan target of 50% efficiency by 2030 must now be regarded as a minimum in the context of enhanced climate ambition and the actions required to deliver on that ambition."