The Taoiseach has said the Defence Forces are sending a multidisciplinary team to Lebanon to assist the UNIFIL investigation over the killing of peacekeeper Seán Rooney.
However, Micheál Martin warned that getting to the truth would be challenging.
Private Rooney, 24, was shot dead while serving with the 121st Infantry Battalion on a UN peacekeeping mission in Lebanon this week while his colleague 23-year-old Trooper Shane Kearney was badly injured and remains in a critical condition in hospital.
Speaking to reporters in Brussels following his final appearance at an EU summit, Mr Martin said: "UNIFIL has already initiated the investigation, the Lebanese authorities, led by a military prosecutor are investigating and our Defence Forces are sending out a multidisciplinary team, also to assist in terms of getting to the truth.
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He said there are always challenges "in situations like this".
"The environment is difficult. The environment is challenging. And as we know, now, with the terrible cost, it's clearly a tense situation," he said.
"These investigations have to be very comprehensive. We are determined to get the truth," he added.
He said Ireland had a "proud and distinguished" service in Lebanon stretching back over many decades.
"These are peacekeepers. We owe it to them and to their families to get to the truth. It is just shocking that a UNIFIL peacekeeping group would be attacked in the manner that they were attacked. And this is something we're determined to get to the bottom of."
A team of eight army personnel will travel from Ireland to Lebanon tomorrow to support members of Private Rooney's battalion.
Members of the team will also assist in the investigation into what happened.
Commandant Gemma Fagan of the Defence Forces Press Office said the team includes four Personal Support Services (PSS) personnel who are trained in critical incident response.
Other members of the team include a legal officer and three members of the Defence Forces Military Police who will assist in the investigation and will be involved in evidence gathering and taking statements.
Commandant Fagan said there are no plans at the moment to bring the injured soldiers home until they are stable enough to travel.
She said Trooper Shane Kearney is still in a critical but stable condition in hospital, and they are liasing with medical staff to ensure he and his colleagues are getting the best possible treatment.
A senior Defence Forces Medical Officer and a chaplain are also with the injured.
A spokesperson for the Defence Forces has said that they are looking at all available options in relation to the repatriation of the body of Private Rooney, including deploying an Air Corps aircraft to the region but this is still in the planning stages.
'A crime against the international community'
UNIFIL today urged Beirut to ensure a "speedy" investigation into the incident.
Spokesman Andrea Tenenti called it "a very serious incident" and told reporters it was "important" for the Lebanese authorities to bring the perpetrators to justice.
"It is a crime against the international community, against peacekeepers who are here... to maintain stability," he added.
It is the first death of a UNIFIL member in a violent incident in Lebanon since January 2015, when a Spanish peacekeeper was killed during retaliatory Israeli fire.
"Peacekeepers are continuing with their activities and patrolling," Mr Tenenti said.
'Unintentional incident'
Wafic Safa, Hezbollah's security chief, told Lebanon's LBCI television that the incident was "unintentional" and called for investigators to be given time to establish the facts.
Relations between UNIFIL and communities in south Lebanon have "always been very positive", the force's spokesman said on Friday.
"The support of the communities is paramount in order for us to implement our mandate."
Lebanon's caretaker prime minister Najib Mikati said that those responsible for the killing "will be punished."
Southern Lebanon is a stronghold for Hezbollah, a powerful armed group and a heavyweight political party backed by Iran.
The party has denied a role in the violence, saying it was an "unintentional incident" between residents and the UN forces.
Minister for Foreign Affairs Simon Coveney said he did not accept Hezbollah's assurances that it had no involvement.
"We don't accept any assurances until we have a full investigation concluded to establish the full truth," he said.
Mr Coveney said two vehicles belonging to the United Nations Interim Force in Lebanon (UNIFIL) and travelling through south Lebanon got separated before the shots were fired.
He had earlier cited "some tension on the ground between Hezbollah forces and UNIFIL in recent months", but said the killing was "unexpected".
Mr Mikati and Lebanon's army commander were at the southern Lebanon headquarters of the UNIFIL to pay their respects.
He said Lebanon remained committed to a 2006 UN resolution that expanded the peacekeeping mission's presence.
Additional reporting: AFP
