Esat Digifone was launched in 1997 as a joint venture between Esat Telecommunications and Telenor of Norway.
After a fallout between Esat and Telenor, BT bought Telenor's stake in 2000, integrating most of its Irish operations under the BT Ireland name. However, BT Wireless, which now owned the Digifone brand (it had been renamed from Esat Digifone in 2000), was not a part of BT Ireland.
In 2001, BT Wireless demerged from BT and took the name mmO2 plc. While mmO2 plc was created in 2001, the O2 consumer brand was introduced on 19 June 2002 in the United Kingdom, Germany, Ireland and the Netherlands (where it lasted for only a year before reverting to Telfort).
O2 Ireland was acquired by Telefónica in February 2006, as were O2 UK and Germany.
Hutchison Whampoa announced its acquisition of Telefónica’s Irish operations on 24 June 2013. The acquisition was completed on 2 March 2015, and the O2 brand in Ireland was retired in favour of the Three brand. All O2 customers were migrated to the Three network.