More than 500 firms of solicitors are co-operating with the Fennelly Commission’s investigation into the covert recording of non-emergency calls in Garda stations.

In an interim report, the commission has set out its work in relation to calls between solicitors and their clients in custody that might have been recorded. It said when it initially requested legal firms to supply telephone numbers to see if they matched with the recordings database, there were only 35 responses.

But a more thorough exercise had resulted in numbers being supplied by more than 500 firms. They were currently being assessed to see if they tallied with numbers from recordings.

The commission has sought an extension of nine months to complete its “time-consuming and labour-intensive” investigation.

The sole member of the commission, retired Supreme Court judge Mr Justice Nial Fennelly, has sought an extension until September 31st, 2016. He had been due to report by the end of December this year.

Mr Justice Fennelly has also sought an extra three junior counsel to assist him in his work, to complement his investigative staff of three full-time and three part-time barristers.

In the interim report published on the Government website, the commission set out the work it has done to date on its investigation of recorded telephone calls over a 30-year period.

Last September,the commission issued an interim report on its investigation surrounding the retirement of former Garda commissioner Martin Callinan.

It is now looking at the wider issue of covert recording of calls in the Garda Communications Centre in Dublin and at each of the 23 divisional headquarters outside Dublin. The time period is between January 1st, 1980 and November 27 2013.

The report said the commission had found no tapes or recordings from the period between 1980 and 1995 and said there were indications the early system mighty have been intended solely to record 999 calls or Garda radio traffic.