The US has asked to join as an 'amicus curiae' or friend of the court, Schrems said in a statement.
The case involves a challenge to Facebook's use of model contract clauses as a legal basis for the transfer of data from its Irish office to the US since the CJEU invalidated the EU-US Safe Harbor framework for data transfers last year.
EU and US officials have negotiated a proposed new framework, the Privacy Shield, to replace the Safe Harbor regime. EU officials, however, have concerns about the Privacy Shield and have been trying to amend the framework to address these.
With the Safe Harbor framework invalidated and the Privacy Shield not yet adopted EU data protection authorities have said that businesses can continue to use other existing data transfer mechanisms, such as model contract clauses, to underpin data transfers to the US for now.
Schrems argues that the use of these clauses does not change the fact that Facebook is still subject to the US mass surveillance program, and that the CJEU has already found them to be in conflict with EU law.
Schrems welcomed the US government request to join the case, saying that this is a chance to "get solid answers".
Unlike in diplomatic conversations with the EU, the spokesperson will not be protected by US laws on confidentiality in the Irish High Court and will be placed under oath, he said.
"The party that gives evidence on behalf of the US government could therefore face severe consequences, if he does not truthfully answer all questions raised on US mass surveillance," Schrems said. "This may be a unique opportunity for us ... I am very much looking forward to rais[ing] all the uncomfortable questions on US surveillance programs in this procedure."
Schrems believes the US is keen to defend its surveillance laws before a European court, as it has previously argued that European courts have misrepresented the laws applicable to US technology companies when data reaches the US, he said.
The American Chamber of Commerce, Business Software Alliance and the Irish Business and Employers Confederation also asked to join the procedure, "as these organisations’ members use the same legal basis to transfer data to the United States as Facebook", Schrems said.
“It would be interesting to see whether EU industry or business associations will ask to intervene also, as, except for large organisations that have binding corporate rules in place for intra-group transfers, model clauses are currently the only workable means in practice for organisations in the EU to transfer personal data to the US legally. The court’s ruling on model clauses will be critical.”
Last month, Ireland's data protection commissioner asked the High Court to refer questions about the use of model clauses to the Court of Justice of the EU (CJEU) "to determine the legal status of data transfers under standard contractual clauses" as part of the case.
Schrems said at the time that he thinks the CJEU would invalidate model contract clauses as a basis for EU-US data transfers.