Natural gas customers in Mayo and Galway are being urged to turn off their supply.
The warning from Gas Networks Ireland came after the operator of the Corrib gas terminal confirmed that a quantity of odourless gas has entered the network.
Natural Gas is odourless, but a smell is normally added for safety purposes.
The problem affects customers in Galway City, Tuam, Headford, Ballinrobe, Claremorris, Castlebar, Westport, Crossmolina and Ballina.
Customers in the affected areas have been told to turn off their gas supply at their meter box until further notice.
Gas Networks Ireland says removing the untreated gas could take two to three days.
It says the risk is low but has apologises to customers for any inconvenience caused
Advice on how to turn off your gas supply can be found here
Additional reporting: Jack Quann