Lawyers for the National Trust, which is attempting to block the development on the north coast, argued that the body responsible for granting World Heritage Site status should have been properly consulted.
A judge was told how UNESCO has the power the remove sites from the designated list.
The planned 365-acre development at Runkerry, to be known as Bushmills Dunes Golf Resort and Spa, received the green light in February last year.
Along with a championship links golf course, the blueprint includes a five-star 120-bedroom hotel and 75 villas.
But the Trust claims the proposed complex encroaches on land zoned in the distinctive landscape setting of the World Heritage Site, on which no development should take place.
The charity argued this was based on a recommendation by UNESCO that there should be a buffer zone to protect the special landscape surrounding the Causeway.
Mr Attwood has indicated he is content for full public scrutiny of his decision.
But opening a judicial review hearing which is expected to last for three days, Stewart Beattie QC, for the Trust, claimed he failed to properly grapple with the potential impact of his decision.
Mr Beattie further claimed that he acted irrationally and was improperly advised on how to deal with the issue.
The complaint of the Trust is that the Minister acted unreasonably by granting (permission) without consulting UNESCO.
Stewart Beattie QC
Developers say the proposed Bushmills resort would create around 360 jobs and a further 300 through suppliers and construction. The investment and advisory team, led by US-based Northern Ireland man Dr Alistair Hanna, have predicted the course and hotel could be ready by 2015.
But the project depends on the outcome of the High Court case. Other grounds of challenge centre on ecological and environmental considerations.
According to Mr Beattie, disquiet was raised within the Northern Ireland Environmental Agency, a branch of the Department of the Environment.
The possibility could not be ruled out that some plant species could be lost as result of development, the court heard.
The rarest of these, the Spring Vetch, was said to be only found at three locations in Northern Ireland.
But the initial submissions on behalf of the Trust focused on UNESCO.
Mr Beattie set out how, during a sitting of the World Heritage Convention Committee in St Petersburg last July, the body requested a halt to development at the Giant's Causeway until an assessment was carried out.
"At the heart of the apparent difference between the Department and the Trust is the contention by the Department that they have complied with their obligations under UNESCO's operational guidelines," he said.
Questioned by Mr Justice Weatherup on the status of these guidelines, Mr Beattie accepted they contained no binding legal obligations.
He stressed, however, that the guidelines were the mechanism by which World Heritage Site status is either granted or lost.
"UNESCO itself can de-list a World Heritage Site," he said.
He added: "The Minister has, in this case, presumed to second guess what the views of UNESCO will be in coming to his decision because he has never had an engagement with UNESCO."
The hearing continues.