The High Court has heard that the choice for Garda Commissioner would be incapable of being independent in the role.
Legal proceedings began in a challenge against PSNI Deputy Chief Constable Drew Harris being appointed as Garda Commissioner.
Ciaran MacAirt, whose grandmother Kathleen Irvine was killed during The Troubles, brought the action in a bid to stop Mr Harris being appointed to the role.
Mrs Irvine was one of 15 people killed by loyalists in the bombing of McGurk's Bar in Belfast in 1971.
Solicitor for Mr MacAirt, Gerard Humphries SC, said there was a clear conflict of interest in Mr Harris taking the role as he is bound by the Official Secrets Act in Britain through his work for the RUC, later PSNI, and his role in the Historical Enquiries Committee.
They also say the conflict is incompatible with the duties of Section 5 of the Garda Siochana Act, in particular regarding State security and the investigation of crime.
"By his involvement with British state security and the Official Secrets Act, Drew Harris cannot independently fulfil the requirements of Garda Siochana.
"Any information that he would have come into by virtue of position in the PSNI, precludes him from discharging his duties in this role."
Mr MacAirt, who is being represented by Kinnear and Co solicitors with the assistance of MacGeehin Toale Solicitors in Dublin, applied for leave for a judicial review of Mr Harris's appointment.
In reference to Mr Harris's experience in the Historical Enquiries Committee, Mr Humphries added that decisions made under Harris neglected measures set out by the European Convention to adequately address instances of collusion.
"In previous roles, he failed and was found to have failed, and now he could be in control of the Gardaí."
The PSNI deputy became the first Garda Commissioner appointed from outside the Republic of Ireland when he was announced as the new chief in June.
He is due to take up the role on September 3.
The challenge is being opposed by the State and An Garda Siochana whose legal team is being led by Reme Farrell SC. They say the application did not meet the required threshold and is a personal gripe by the applicant.
"My submission is quite simply that the application is un-stateable, and amounts to no more than a personal view on behalf of the applicant.
"No attempt has been made to address the judicial review application, this is nothing more than a series of personal views dressed up as something else.
"An applicant is not entitled to come to court to say; 'The government has made the wrong decision and I want my view of matters to be substituted for the view of people in charge of law-making decisions'."
Mr Harris is a former Royal Ulster Constabulary (RUC) officer and his officer father Alwyn was killed by an IRA bomb in 1989.
After today's proceedings, Mr MacAirt said it was his right as an Irish citizen to point out flaws in an appointment.
"I have great fears for the basic human rights of Irish citizens if Harris is placed in charge of An Garda Siochana, because of what he has done in the last 15 years with respect to victims and survivors of the Troubles.
"He has drawn out cases in court, reducing resources for the police ombudsman, stymieing the amount of information families can get from the police."
The McGurk's bar bombing was one of the worst atrocities of the Troubles and the families of those who died have long campaigned for a fresh inquest.
The bombing was carried out by loyalists but at the time security forces blamed the IRA, prompting speculation the dead might have included IRA members who were carrying the device.
Campaigners discovered new evidence not heard at the original inquest held the year after the bombing and have rejected a number of committee reports, which they say ignores the issue of collusion by state forces and loyalist paramilitaries.
Justice Denis McDonald says he hopes to have a decision for the court on Wednesday at 11am.