Ten parties, including the US government, have made arguments to the Commercial Court as to why they should be allowed to have a watching brief in a case being taken by the Data Protection Commissioner Helen Dixon.
Ms Dixon is seeking to have the question of the legality of a mechanism known as "Standard Contractual Clauses" or SCCs, which enables the data of Facebook users in the EU to be transferred to the US, tested in the European courts.
The question of the legality of SCCs is of great importance not only to Facebook, but also to thousands of other US technology companies operating in Europe.
The case arises from a complaint made about the extent to which Facebook protects data it transfers to the US under an EU-US agreement known as "Safe Harbour", taken by Austrian privacy campaigner Max Schrems.
The Office of the Data Protection Commissioner initially declined to investigate Mr Schrems complaint, as it claimed it did not have the authority to do so.
Mr Schrems then appealed that decision to the High Court, which referred it on to the European Court of Justice.
Last year it struck down the Safe Harbour agreement, because it found the data was not being adequately protected in the US.
Mr Schrems was then allowed to amend and resubmit his complaint to the Office of the Data Protection Commissioner, which is now investigating it.
His new complaint questions the legality of SCCs which are an alternative method which Facebook is using to transfer data to the US in the wake of the Safe Harbour ruling.
The court heard that Commissioner Helen Dixon has made a draft decision finding that a number of Mr Schrems concerns are "well-founded".
As a result, the Commissioner has asked the Commercial Court to refer the issue of the legality of SCCs to the European Court of Justice, so that it can make a judgment on them.
In total, ten parties have applied to become "amicus curiae" or friend of the court, which would allow them to participate in a limited way in the Irish court proceedings.
The parties include the US government; industry organisations IBEC, the Business Software Alliance (BSA) and Digital Europe; human rights and civil liberties organisations including the Electronic Frontier Foundation, Electronic Privacy Information Center, Irish Council for Civil Liberties, the American Civil Liberties Union and the Irish Human Rights and Equality Commission; and a lay litigant Kevin Cahill.
In his opening remarks, counsel for the Data Protection Commissioner Michael Collins said Helen Dixon's overarching concern is that the case remains manageable, and that there was an extraordinary number of applications for amicus curiae status where there could be overlap.
He said the courts have consistently ruled that amicus curiae is a measure to be sparingly exercised and that any parties given that status must have a public policy role and be able to assist the court with expertise or information others can't.
He claimed their role must be limited, that they wouldn't be a party to the proceedings, that they shouldn't be allowed make oral submissions, that they must bear their own costs and have no right of appeal.
He also suggested that as the court goes through the proceedings it may be necessary to amend the issue to accommodate developments in the formation of Privacy Shield, a replacement agreement to Safe Harbour currently being worked on by the EU and US.
In his submission, counsel on behalf of the Business Software Alliance, Maurice Collins, said his clients do not intend to assist or oppose or act as a cheerleader to the parties to the case in any way.
He said if SCCs were to ultimately be struck down, it would have significant economic consequences for a range of companies across the EU.
He said it was estimated that an interruption in a free flow of data across the Atlantic could knock 0.8-1.39% off the annual GDP of the EU, with tremendous commercial consequences, particularly for BSA members.
BSA members had revenues of $559bn in 2015, $133bn of which is attributable to work in the EU, he stated.
He added that any invalidation of SCCs would not only inhibit external transfers to the US, but could also impact cloud based business carried out within the EU.
The BSA would bring particular balance and expertise, with offices in 15 countries and having been a part of 40 related cases in the US courts, he claimed.
Counsel for the US government said it is difficult to exaggerate the importance with which it sees the case.
In her submission, Eileen Barrington said the US authorities view the case as being of critical importance and so see it as critical that this court and the European Court of Justice have its input.
She said it was vital that EU member states have an understanding of the position and views of the US government.
Ms Barrington said Facebook had consented to the US government being made an amicus curiae in the case, and had stated that of all the applicants the US government has the strongest case to be allowed.
She added that the Data Protection Commissioner had adopted a formally neutral view, but had accepted in correspondence that the issues the case raise are of great importance to the US.
Mr Schrems has no objection to the US government being included, she said, subject to the conditions that others also be included and that the US should confine itself to a particular role.
She said the US had a bona fide interest in the proceedings, and would bring a particular expertise in the area of US law, and the under negotiation successor agreement to Safe Harbour, Privacy Shield.
She said in its submissions, the US government would describe the US regime and law in the area.
She admitted that the US government could not be described as an entirely disinterested party, but added that case law has moved on to a point where the fact it has an interest does not preclude it being an amicus.
In his submission to the court, counsel for IBEC said a whole variety of actors are affected and interested in the outcome of the case.
He said the case was of international transcendent legal and commercial importance, and an entire chain of economic activity on this island is dependent on it.
IBEC brings a perspective that no others can, he argued.
It has a nominee on the National Data Forum, represents IT giants here but also 185,000 SMEs, many of whom are ultimately involved in processing and controlling personal data in reliance on SCCs.
He said although Facebook is a member of IBEC, it has had no involvement in the business organisation bringing that application, and it will not if IBEC is given amicus curiae status.
All IBEC is asking permission to do, he added, was make a written submission.
Court told all legal, technical information must be put before ECJ if case is refered on
Senior Counsel Michael Cush for Digital Europe told the court that if it refers the case on, it must put before the European Court of Justice (ECJ) all the legal and technical information relevant to the case.
It is not for the ECJ to assemble that itself, he said.
He said the organisation he represents is best placed to bring a perspective on EU law in this area, as it has a presence in every member state its members are engaged day to day in this activity and in domestic regulatory schemes.
Digital Europe is, Mr Cush said, the most substantial representative organisation for the digital technology industry in Europe, and is being upfront in declaring that it is not impartial when it comes to this case.
Counsel for the Electronic Frontier Foundation, Ronan Lupton, said it is a US not-for-profit law firm dedicated to defending civil rights in the US.
He said if the foundation were made an amicus curiae it could bring a perspective to the case free from commercial constraint.
He said it has the consent of defendants Max Schrems and Facebook, while the Data Protection Commissioner was taking a neutral stance on its application.
Michael Lynn, senior counsel for the Irish Council for Civil Liberties and the American Civil Liberties Union (ACLU), said his clients have no interest in the outcome of the case, but are public interest bodies. He said the ACLU has enormous experience as an amicus curiae in other related cases, including involving US surveillance law and practices.
Counsel for the Irish Human Rights and Equality Commission said it has a statutory power which allows it to apply for amicus curiae status in civil or human rights cases.
The counsel said the commission is also a member of two international human rights network, which will enable it to obtain information and express positions which may be useful to both the Commercial Court and the ECJ if the case ends up there.
She also said the commission has a history of intervening in an amicus capacity in human and civil rights cases before the national courts and the ECJ.
Counsel for the Electronic Privacy Information Center (EPIC), a Washington based not-for-profit NGO that specialises in privacy and surveillance issues in the US, also applied for amicus status.
He said EPIC has been involved in 85 superior court cases in the US around privacy issues, including the recent one involving Apple and the FBI. He said it has a strong knowledge of the key matter in the case, which is surveillance.
Responding to the applications, counsel for defendant Facebook, said SCCs are important for Facebook, but it has other mechanisms it can use instead.
But there are other parties not involved in the case for whom SCCs are vital to enable them to transfer data beyond the boundaries of the EU and that are vital to their business, he added.
He said the extent of the effect on trade and business if SCCs were ruled invalid is conservatively 1% of the EU's GDP of €14.3 trillion, or €143bn on an annual basis.
He said many firms could not conduct business if data transfers outside the EU were stopped.
He added that it would not just be US companies that would be impacted - all firms operating in the EU would be barred from transferring data to all bar 20 countries in the world.
As a result, he said, the possible commercial impact of a finding that SCCs are invalid would not only be enormous for Ireland, but for Europe as a whole.
He also claimed that a number of fundamental rights are central to this case.
Mr Justice Brian McGovern said he hoped to rule on the applications before the end of the month.