The Commission for Regulation of Utilities has said that odourless gas was pumped into a gas network in Co Mayo last year following a computer system upgrade.
The finding was made in a report published by the CRU into what caused the gas to be pumped into the network at the Corrib gas refinery in Bellanaboy.
The incident had required 10,000 homes and business in Mayo and Galway to switch off their gas for three days.
The report said that procedures have been put in place to ensure it will not happen again.
The system was shut down to facilitate the upgrade, and caused a different computer system to go into default mode.
This other system manages the injection of a chemical constraining sulphur, which gives natural gas its smell. Once the system went into default mode, it stopped injecting odourants into the gas.
However, the procedures for the start-up process after the system upgrade on 21 September did not include a step requiring the operator to reset the odourant injection system to inject odourant.
Shell EP Ireland identified that odourless gas had been injected in the gas network system at 2am, and by 7am the natural gas emergency plan was activated.
10,000 homes and businesses in Mayo and Galway were asked to turn off the gas supplies at the meter, while all affected gas was directed back to the terminal and flared off to remove the non-odourised gas from the system.
There had been no additional risk of a gas escape as a result of the incident.
However, there were significant additional safety concerns because it would not have been possible to detect odourless gas it by smell, as natural gas has no smell, colour or taste.
Today's report says that following its investigation, enforcement actions were taken by the CRU against both Shell EP Ireland and Gas Networks Ireland.
It also said the deficiencies in operating procedures that led to the incident have been rectified.