Lawyers for the State claim a decision on the retention and access of mobile phone data could affect the investigation of serious crimes.

The Supreme Court has begun hearing their appeal against a successful legal challenge taken by convicted murderer Graham Dwyer.

Mobile phone data used to track various phones proved crucial in the successful prosecution of Dwyer in 2015.

Four years ago, the Cork-born architect was found guilty of murdering childcare worker Elaine O’Hara in 2012.

Dwyer is yet to appeal his conviction – but last year he won a significant case against the law which allowed his phone data to be retained and accessed.

The High Court found it contravened EU law, but the State claims the act is compliant and the Supreme Court has begun hearing a fast-tracked appeal.

Counsel for the State Paul Gallagher said there was an enormous amount at stake in this case, not only in relation to Dwyer’s conviction but more generally as to the powers of the State in retaining and accessing metadata.

He said if it was allowed to stand, it would create a virtual space in which criminal activities could exist and not be accessed by State authorities.

What is critical, he said, is the protection of citizens’ rights by ensuring access is done on the correct basis and only when there’s a high degree of suspicion

Mr Gallagher claimed the law in question meets the relevant criteria.

Reporting from Frank Greaney at the Supreme Court