Foreign Affairs Minister Simon Coveney says scenes from the Ukrainian suburb of Bucha could be repeated elsewhere as Russian forces withdraw.
Images emerged on Sunday of Ukrainian civilians lying on the streets of the town north-west of Kyiv, with witnesses saying the victims were killed by Russian forces without any apparent provocation.
Some of the victims had their hands tied and were shot in the back of the head, Ukrainian authorities said.
Ukrainian officials laid the blame for the killings squarely at the feet of Russian troops, with Ukrainian President Volodymyr Zelenskyy calling them evidence of "a genocide".
Ukraine's Foreign Minister, Dmytro Kuleba, also posted some of the graphic images on Twitter.
Bucha massacre was deliberate. Russians aim to eliminate as many Ukrainians as they can. We must stop them and kick them out. I demand new devastating G7 sanctions NOW:
-Oil, gas, coal embargo
-Close all ports to Russian vessels and goods
-Disconnect all Russian banks from SWIFT pic.twitter.com/oZkCAETCQp
— Dmytro Kuleba (@DmytroKuleba) April 3, 2022
Minister Coveney spoke to Minister Kuleba on Sunday. He told Newstalk Breakfast: "He was quite graphic in relation to what he's seen and it's awful.
"They've already invited in a team from the ICC Prosecutor - that's the International Criminal Court - to actually document and put a case together in relation to war crimes and crimes against humanity.
"So they are effectively inviting outsiders in to look at what's happened.
"And I think, not for the first time, we are seeing what are likely to be determined as war crimes as part of this war.
"A lot of civilians dead, bodies lying on the streets, women and girls having been raped and killed - this is about as awful I think as you can imagine."
And he says such scenes may be replicated in other places taken back by Ukrainian forces.
"It's effectively in the outer suburbs of Kyiv, in a place called Bucha.
"But what's even more horrific I think is that this is potentially what we may be able to expect from some of the other towns that are effectively regained by Ukrainian forces."
Asked if he is in any doubt that these are war crimes being talked about, he says: "I'm not, but it's not up to me to determine that.
"But if you're asking me for my opinion as to whether I think there was more war crimes committed here from what I've heard, the answer to that question is yes, and from what I've seen in terms of photographs the answer to that question is yes.
"That will be up to the International Criminal Court and the team there - as well as other entities".
More EU sanctions
And Mr Coveney says a fifth package of European Union sanctions against Russia will be agreed in the coming days.
"There will be conversations taking place today and tomorrow and later on this week in terms of a fifth sanctions package.
"Ireland will be pushing hard for a significant ratcheting up of sanctions beyond where we have gone to date.
"That includes energy, that includes the access to ports potentially, and it includes more Russian banks being removed from SWIFT.
"And whatever else is appropriate... but certainly Ireland will be at the sharper end in terms of what we're asking".
He adds that just under 18,000 Ukrainians have arrived in Ireland as of Sunday night.
"This is going to put Irish society under pressure, but it is something that we need to respond to", he says.