Public health expert Dr Gabriel Scally is calling for governments North and south "to declare an all-island emergency" to tackle rising Covid cases on both sides of the Border.
The Belfast-born medic has warned a failure to act before Christmas "will have consequences" and could lead to more deaths across the island.
"While people may make merry at Christmas, they may not be so merry in January or February if they are burying friends, relations and neighbours because of a couple of days' indulgence… We are in a really dangerous situation," he said.
Case numbers are rising in both jurisdictions, with Dr Scally citing a lack of political leadership and low compliance with regulations as the reasons for increased infections, hospital admissions and deaths.
"We would be in a completely different place if the North and south had an agreement to keep the virus under control in the summer to eliminate it from the island - which was easily doable at that time. It was a deliberately missed opportunity," he said. "Deaths will go on until the virus is no longer with us; the only time you don't have deaths is if you don't have circulating virus. We were in that situation last summer and it was squandered by politicians."
A memorandum of understanding was agreed by the Northern and southern chief medical officers in April which aimed to strengthen co-operation in response to the pandemic. It also provided a formal framework to manage the transfer of patients between jurisdictions if critical care capacity is overwhelmed in either jurisdiction "should the need arise".
"The understanding they have had is to do nothing," said Dr Scally. He wants the Taoiseach and Northern Ireland's First Minister Arlene Foster to "recognise geography, understand Covid does not recognise borders and introduce an integrated response" as it is the "only way of saving lives and getting us through the next difficult few months. I would appeal to them as a matter of urgency that they meet and declare that it is an all-island emergency and there will have to be an all-island response and a proper finding, testing, tracing support system in both parts of the country."
Dr Scally said there has been "political support and political encouragement for people to celebrate Christmas by meeting together in ways that are completely unacceptable".
"There is no more lethal mixture than that. Pleasant as it may be at the time, it could have appalling consequences, particularly in the North.
"There are no wise men coming providing advice from Westminster; that is all nonsense and they [Stormont leaders] need to stop listening to what Downing Street are saying and they need to start making decisions in the interests of their own population."
He said the Executive "doesn't seem to be able to make decisions and has failed to do what needs to being done".