Among the municipality companies that have merged into what is now the Iren Group, the first to come into existence is the Officina del Gas e dell’Elettricità (Workshop of Gas and Electricity), which has been lighting Reggio Emilia since 1 January 1903, even anticipating the "Giolitti Law" on municipality companies by a few months.

The City of Parma incorporated the new course introduced by the "Giolitti Law" and, following a city referendum, established the Azienda Energetica Municipale (AEM) (Municipal Energy Company) in 1905.

In 1907, the Azienda Elettrica Municipale (Municipal Electric Company) was founded in Turin for the production and distribution of electricity. In the 1980s the company would change its name to Azienda Energetica Municipale (AEM) and in 1997 to Azienda Energetica Metropolitana (AEM S.p.A.).

On 1 November 1922, the Municipality of Genoa established the Azienda Municipale del Gas (Municipal Gas Company) of Genoa, taking over the direct management of gas production and distribution.

With the gas company being entrusted with the management of the civic aqueduct and other facilities inherited from the annexed municipalities in Greater Genoa, the Azienda Municipalizzata Gas e Acqua (AMGA) (Municipalised Gas and Water Company) was born, opening in Genoa the experience of the public multi-service in gas and water, later extended to waste collection, local transportation, and urban maintenance.

The municipality of Reggio Emilia acquired SEEE (Società Emiliana Esercizi Elettrici), to which the gas and electricity production and distribution facilities had been sold in 1924, and in 1962 established the Azienda Municipalizzata Gas (AMG). The new course begins: "Methane for All".

By 1950, Parma's utilities had been combined into one entity: the Azienda Municipalizzata Elettricità Acqua Gas (AMEAG). In 1965, AMEAG was transformed into the Azienda Municipalizzata di Pubblici Servizi (AMPS).

On 1 January 1969, the Muncipality of Turin established the Azienda Municipale Raccolta Rifiuti (AMRR) (Municipal Waste Collection Company), entrusted with the service of collecting, transporting and disposing of urban waste in Turin.

The Azienda Municipalizzata Nettezza Urbana (AMNU) (Municipalised Waste Collection Company) of Piacenza began operations in 1972 with the sole responsibility of providing waste collection, and expanded its services over the following decades. In 1988, the company would become the Azienda Servizi Municipalizzati (ASM) (Municipalised Services Company).

In the decade following the establishment of the Reggio Emilia Azienda Municipalizzata Gas (1962), the company was reorganised into an inter-municipal consortium, incorporating the water distribution service as well. Thus, on 1 January 1974, the Azienda Gas Acqua Consorziale (AGAC) (Consortium Company of Water and Gas) (AGAC) was born.

With the new millennium, it becomes necessary to strengthen local utility companies so that they can resist competition from private operators. On 24 February 2005, the municipalised utilities of Reggio Emilia (AGAC), Parma (AMPS) and Piacenza (TESA) merged, creating ENÌA S.p.A., an ecoutility structured as a holding company.

On 31 October 2006, IRIDE SpA was born, the result of the merger of AMGA Genoa and AEM Turin, thus creating an articulated public-private system that constitutes the synthesis of the historical bipolarity of the cities of Genoa and Turin and effectively closing a century of municipalised utility history.

On 1 July 2010, the merger project between ENÌA and IRIDE was completed and IREN SpA was born.

The multi-regional group becomes a nationwide multi-utility, among the top operators in district heating, as well as a leader in the water sector, environmental services, gas and electricity.

In 2015, Azienda Multiservizi Igiene Ambientale (AMIAT) of Turin, a new acronym for AMRR since 1990, joined the Group, thus increasing its national relevance in waste management and being able to manage the entire industrial waste chain in also Piedmont in an integrated way.

On 1 January 2016, IRETI was established with the goal of integrated, nationwide management of electricity and gas distribution and integrated water service. The activities previously the responsibility of the territorial distribution companies flow into the company.

In 2016, joining the group were TRM S.p.A., the Turin waste-to-energy company, and ATENA S.p.A., the company into which Vercelli's two municipal utilities have been merged since 1999: Azienda Autonoma dei Servizi Municipalizzati (AASM) (Autonomous Municipal Services Company) and Municipalizzata Nettezza Urbana (AMNU) (Municipalised Waste Collection). In 2018, ATENA became the Azienda Servizi Municipali (ASM) (Municipal Services Company).

On 11 April 2018, the acquisition of the ACAM Group, active in La Spezia and Province, is finalised through the acquisition of ACAM S.p.A. by IREN S.p.A.

Iren Rinnovabili (Renewable) becomes Iren Smart Solutions and extends its activities to all areas of energy efficiency, with focus and innovative solutions aimed at the Public Administration, Businesses, Apartment complexes and Third Sector entities.

On 8 January 2019, IRETI signed the contract regarding the transfer of ownership of 100% of the shares of the company Busseto Servizi S.r.l, operator of the methane gas distribution service in the Municipality of Busseto with 3,059 redelivery points present on a total of 90 km of network.

On 19 June 2020, Iren Ambiente S.p.A., a wholly-owned subsidiary of Iren S.p.A., acquired the Enlarged Environment Division of Unieco S.C.L.C.A., whose activities are spread over a territory encompassing five Italian regions: Piedmont, Emilia Romagna, Marche, Tuscany and Puglia.

The large multi-regional group into which the DNA of the municipal utilities has now converged is celebrating ten years of life and more than 100 years of history. A first milestone for a company that now has a turnover of more than 4 billion euro, 8,000 employees and provides energy and services to more than 7 million citizens.

Iren enters the Bloomberg Gender Equality Index (GEI) 2021 for the first time. GEI provides transparency in the gender practices and policies implemented by publicly traded companies, thereby deepening and broadening the environmental, social, and governance (ESG) data available to investors and the financial community.

Iren has been certified in 2021, for the fourth consecutive year, as Top Employer Italia, the official recognition of the Company's excellence in HR policies and strategies and their implementation to contribute to people's well-being, improve the work environment and the world of work.