From the wikipedia Original historic use of the plural form: use of the singular form is a relatively new habit. Even at the beginning of the twentieth century, speakers had the choice of using the plural form ... though na Ceithre Chúirt would be more correct according to current language rules.

The lads in the courts would be a stickler for An Caighdeán Oifigiúil, the county councils mainly use Google Translate now so likely same with the TFI.