Phone data was used to "build up an intensely detailed picture of every aspect of Mr Dwyer's life," his lawyers told the court
Remy Farrell SC, for Dwyer, told the three-judge Court of Appeal that mobile phone data should not have been admitted as evidence in his client's trial as the retention of that data was a breach of his rights under the European Charter of Fundamental Rights.
Mr Farrell said that the Court of Justice of the European Union (CJEU) has said "again and again" that the retention of mobile phone data cannot be done and the Irish courts must now "internalise" what the European court has said.
Dwyer (50), a Cork-born architect with an address at Foxrock in Dublin, was convicted by a jury at the Central Criminal Court in 2015 of the murder of Elaine O'Hara on August 22, 2012.
His victim had been discharged from a mental health hospital hours earlier. Dwyer fantasized about stabbing a woman during sex and used Ms O'Hara to fulfil his fantasy.
After murdering her he disposed of some of her belongings in the Vartry reservoir in Wicklow and tried to make it look like she had committed suicide. He dumped her body in a forest where it was found in 2013.
He was led into court today by prison officers and was wearing a dark grey suit, black tie and white shirt.
He occasionally put on glasses as he entered notes into a folder he had brought with him.
Much of the evidence at Dwyer's trial focused on text messages between a "slave" phone used by Ms O'Hara and a "master" phone used by Dwyer and on the movements of those phones.
Mr Farrell said today that he is objecting only to the retention of data in relation to a phone with a number ending in 407 which was referred to as Dwyer's work phone.
He said information from that phone was used by the prosecution to attribute the other phones to his client but that evidence should not have been admitted.
He said that a 2011 law that required mobile phone companies to retain data relating to the use and movement of mobile phones for two years, amounted to "general and indiscriminate" surveillance and did not target people suspected of criminal activity but "everyone who has a mobile phone".
Dwyer, he said, was not under suspicion at the time that his mobile phone data was retained but that information was then used to "build up an intensely detailed picture of every aspect of Mr Dwyer's life."
Mr Farrell said that "everything the Court of Justice is talking about" in relation to the dangers of data retention had happened in this case.
Mr Farrell said there may be "considerable misgivings" in the Irish courts about the CJEU's findings and, he said, there may be historical reasons why European Union member states such as Germany and Austria had been to the forefront of asserting data rights.
But, he said, "one has to accept the result and then fully internalise the result because all State institutions, particularly the courts, are under a duty of loyal cooperation insofar as the Court of Justice is concerned."
Mr Farrell is continuing his submissions this afternoon in front of the president of the court, Mr Justice George Birmingham, Ms Justice Isobel Kennedy and Mr Justice John Edwards.