Mr McGuill said he did not believe Nash should be involved "at a sideshow" where he could face accusations against which he would be unable to defend himself.
He argued that the only relevant material which Nash could give to the inquest was already prohibited under Section 30 of the Coroners Act. It relates to questions of criminal liability which is specifically excluded from
being examined at inquests.
Dr Farrell said he had taken the decision to issue a summons to Nash to attend the hearing because Solicitor James McGuill said the intention of Dublin City Coroner Brian Farrell to call his client as a witness had placed Nash "in a particularly vulnerable position".
Mr McGuill was speaking on the second day of the resumed hearing into the deaths of Mary Callinan, 61, and Sylvia Shields, 59, at the Coroner's Court in Dublin yesterday.
The women, who were psychiatric patients at Grangegorman, were found brutally murdered in their sheltered
accommodation close to the hospital grounds on March 6, 1997.
Nash, from Huddersfield, England, is a principal suspect for their murder after he made a statement, which he later retracted, outlining details about the Grangegorman killings which were known only to gardaí.
Nash is already serving a double life sentence in Arbour Hill Prison for the murder of his girlfriend's sister, Catherine Doyle, and her husband, Carl, in Roscommon in August 1997.
Yesterday, he felt he might have evidence which could help the inquest.
"I put it no higher than that," Dr Farrell said.
"It would not go to any question of criminal liability," he said.
The coroner said Nash had written a letter to him on October 21 via the Governor of Arbour Hill Prison in which the prisoner had stated that he was willing to provide the inquest with a written statement, although he was concerned about his personal safety.
In reply, Mr McGuill said that Nash could not be considered an essential witness under the terms of a Supreme Court ruling in relation to the ability of coroners to order witnesses to attend inquests.
He also pointed out that there was already a disputed statement involving Nash over the Grangegorman murders, although his client had never been formally charged with any related crime.
"I would advise Mr Nash not to discuss that statement in this forum," Mr McGuill said.
He said no other parties should be allowed to use the Coroner's Court to obtain evidence from Nash, if criminal proceedings were pending.
Paul Coffey SC, for the Garda Commissioner, said the gardaí were concerned that proceedings at the inquest might have the effect of "immunising" Nash from any future criminal prosecution.
"The court should be slow to embark on anything that might impair a fair trial," Mr Coffey said.
Dr Farrell reserved his ruling on the issue of Nash's attendance until November 14. He also adjourned the inquest until November 20, when further evidence from garda witnesses will be heard.