Natural gas customers in Galway city, Headford, Castlebar, Tuam Ballinrobe, Claremorris, Westport, Ballina and Crossmolina are being told to turn the supply at the meter box off until further notice.
Gas is an odourless substance until a smell is added so it is noticeable.
If a gas leak was now to occur, homeowners would not be able to smell it.
It was processed at the Ballanaboy terminal in Mayo however the odourless gas managed to enter the network, Shell EP confirmed.
In a statement issued by Gas Networks Ireland, the company confirmed a ‘quantity of odourless gas’ entered the network.