This is Galway Co. Co. using the LOCAL GOVERNMENT (SANITARY SERVICES) ACT, 1948. to try and stop motorhomes from parking. As the name of the legislation indicates it's a public health issue which is being addressed, no one wants temporary dwellings erected or placed without adequate facilities to cater for outflows of sewage or waste water.
Modern self contained motorhomes do not present the issues the LOCAL GOVERNMENT (SANITARY SERVICES) ACT, 1948. was enacted to control.
Ignore the sign.