The EU is taking Ireland to court in a row over the fuel used for private yachts.
The European Commission is taking action because Irish law allows yacht owners to use cheaper "green" diesel.
Brussels says this cannot be allowed because it lets Irish vessels use the same fuel that was meant for commercial vessels.
Ireland's law also means Irish sailors could be fined in other countries, if they are using cheaper fuels than those countries allow.
The authorities say Ireland has ignored letters sent earlier this year, warning about the problem.