This was the firemark used by the General Insurance Company of Ireland.
To insure a building, people paid a premium to an insurance company. These companies made a lot of money from the scheme and used it to start their own fire brigades.
Firemarks were used to distinguish between, and advertise, different insurance offices, and were placed on the outside of an insured building.
Brigades would use the firemarks to identify whether a certain building was insured by them. When a building was on fire, several brigades would attend and if they did not see their specific firemark on the building, they would go away and leave it to burn.