The High Court has been told that Drew Harris ‘lacks the independence’ to fulfill the State’s top policing role.
The former deputy Chief Constable with the PSNI is due to take up the position of Garda Commissioner next month.
However, the grandson of a woman killed in the 1971 bombing of McGurk's Bar in Belfast is seeking leave for a judicial review of the appointment.
Lawyers for Ciarán MacAirt told the court that giving the former RUC officer the role is a breach of Irish national security.
Senior counsel for the State, however, argues it amounts to a personal opinion.
Judge Denis McDonald is expected to make a decision tomorrow morning.
Mr MacAirt spoke briefly outside court this evening.
Ciarán McAirt (left) and his solicitor Niall O Murchú. Image: Paul Quinn
He observed: "It will be very interesting tomorrow to hear with that judgement is, but I still stand by the original application.
"I have great fears for the very basic human rights of Irish citizens if he is placed in charge of An Garda Síochana."