Ryanair has described as "absolutely outrageous", and "nonsensical" the Government measures advising people to quarantine for 14 days when they come back from a non-green list country.
The airline is challenging the measures, saying they have had a devastating impact on it and on Aer Lingus.
Lawyers for Ryanair told the High Court that travel advice usually takes the form of advising people not to travel to particular locations, because of danger or disease. But senior counsel Martin Hayden said this was an expansion of the usual travel advice.
He said the State claimed the quarantine measures were just advice. But Ryanair says they involve restricting people's entitlements and freedoms and it says people who do not follow the advice suffer consequences.
Mr Hayden said the State had the ability to bring in legislation to underpin advice but had not done so in this case.
He suggested this was to ensure the measures would be immune from oversight and could not be legally challenged or alternatively, was down to incompetence.
He said the Government was giving the impression that the travel quarantine measures were mandatory.
Even in recent days, the leader of the Green Party, Eamon Ryan, had said the measures were mandatory while the latest online information made it more obvious that they were advisory, he argued.
Different ministers and different departments were saying contradictory things, Mr Hayden said. He said civil servants had to quarantine if they returned from a non-green list country and lost pay as a result.
It was not true that there was no impact on people who decided not to quarantine. He also said it was not clear what constituted essential or non-essential travel.
He said travel advice was being converted into what it could never have been designed to do, in a nonsensical manner. He said the State's response was "institutional dissembling".
Mr Hayden said Ryanair was not looking for further regulation or legislation but wanted a clear and level understanding about what people can and cannot do.
If State bodies internally were confused, he said, then the public must be even more confused.
He agreed with Mr Justice Garrett Simons that the Department of Foreign Affairs had recently removed the word "required" from its online travel advice on quarantine, but he said the advice was still being portrayed as mandatory by other Government departments.
Mr Hayden said he was not saying that the Government could not give advice. But he said the present scenario was not "mere advice" but had a coercive effect.
He said it was a publication for the public, so the Government should be clear about the messaging it wants to give.
Mr Hayden said the restrictions were about public health, not about an individual's health and he said very high profile people had been penalised for travelling abroad.
He said state and semi-state bodies all lived in the world of these measures being mandatory.
He said it was a matter of extreme seriousness to the airlines and to the citizens of Ireland.
Mr Hayden will finish addressing the court on behalf of Ryanair tomorrow. Lawyers for the State will then reply. The case was expected to last 2 days but may take longer.
Earlier, lawyers for Aer Lingus, which is a notice party to the case, said it was an extremely important case for the airline whose business had gone down by millions due to the advice, not the pandemic.
Mr Hayden said the airlines were bitter rivals but both were suffering a devastating impact.