The Department of Foreign Affairs has said it condemns "in the strongest possible terms the execution of four pro-democracy figures by the military regime in Myanmar".
It stated: "These cruel and inhumane killings are renewed evidence of the ongoing brutality of the Tatamdaw."
Myanmar's ruling military announced today it had executed four democracy activists accused of aiding "terror acts", sparking widespread condemnation of the country's first executions in decades.
Sentenced to death in secretive trials in January and April, the men were accused of helping a civilian resistance movement that has fought the military since last year's coup and bloody crackdown on nationwide protests.
Among those executed were democracy campaigner Kyaw Min Yu, better known as Jimmy, and former lawmaker and hip-hop artist Phyo Zeya Thaw, an ally of ousted leader Aung San Suu Kyi.
The two others executed were Hla Myo Aung and Aung Thura Zaw.
State media said "the punishment has been conducted", but did not say when, or by what method.
Previous executions in Myanmar have been by hanging.
The Department of Foreign Affairs said: "Ireland extends its heartfelt condolences to the families of Phyo Zeya Thaw, Kyaw Min Yu (Ko Jimmy), Aung Thura Zaw, and Hla Myo Aung.
"Ireland remains strongly opposed to the use of the death penalty in all cases and in all circumstances. It constitutes cruel, inhuman and degrading treatment and is contrary to the right to life.
"Ireland calls on the military to release all those arbitrarily detained in Myanmar and once again condemns the politically motivated convictions of other detainees."
Young demonstrators today shouted slogans and hold banners during an anti-coup protest in Yangon, Myanmar, following the executions.
EU foreign policy chief Josep Borrell also "strongly" condemned the Myanmar junta's "politically motivated" executions of four prisoners.
"The European Union strongly condemns the executions," Mr Borrell said in a statement.
"These politically motivated executions represent yet another step towards the complete dismantling of the rule of law and a further blatant violation of human rights in Myanmar."
The shadow National Unity Government (NUG), which is leading efforts to undermine the junta's attempts to rule Myanmar, said it was time for an international response.
"The global community must punish their cruelty," said Kyaw Zaw, a spokesperson for the NUG president's office.
Myanmar has been in chaos since the coup, with the military, which has ruled the former British colony for five of the past six decades, engaged in battles on multiple fronts with newly formed militia groups.
United Nations human rights chief Michelle Bachelet called the executions a "cruel and regressive step" by the military that would "only deepen its entanglement in the crisis it has itself created."
Human Rights Watch acting Asia director Elaine Pearson said it was "an act of utter cruelty."
"The junta's barbarity and callous disregard for human life aims to chill the anti-coup protest movement," she added.
The executions were the first carried out among some 117death sentences handed down by military-run courts since the coup, according to the Assistance Association for Political Prisoners (AAPP), which has been tracking arrests, killings and court verdicts in Myanmar.
Families of the executed men were denied the opportunity to retrieve their loved ones' bodies, said Thazin Nyunt Aung, wife of Phyo Zeyar Thaw, comparing it to murderers covering up their crimes.
"This is killing and hiding bodies away," she told Reuters. "They disrespected both Myanmar people and the international community."
Nilar Thein, wife of Kyaw Min Yu, said she would hold no funeral without a body. "We all have to be brave, determined and strong," she posted on Facebook.
The men were held in Yangon's Insein prison, where families visited last Friday, according to a person with knowledge of the events, who said prison officials allowed only one relative to speak to the detainees via video call.
The junta made no mention of the executions on its nightly television news bulletin on Monday. Its spokesperson last month defended the death sentences as justified, and used in many countries.
The party of Ms Suu Kyi said it was "devastated" by the executions.
"The National League for Democracy is devastated," it said in a statement condemning the "outrageous crime" committed by the junta.
The United States vowed to work with regional allies to hold the military accountable and called for a cessation of violence and release of political detainees.
"The United States condemns in the strongest terms the Burmese military regime's heinous execution of pro-democracy activists and elected leaders," a White House National Security Council spokesperson said in a statement.
France condemned the executions and called for dialogue among all parties, while Japanese Foreign Minister Yoshimasa Hayashi said the executions would further isolate Myanmar.
The executions have shattered hopes of any peace agreement, said the Arakan Army (AA), one of more than a dozen ethnic minority armies in Myanmar that have fought the military for years.
