Either way, the cold snap is set to continue over the next few days as temperatures plummet to well below normal for this time of year.
Snow is expected in higher parts of eastern counties as the icy chill takes hold.
"We are probably going to see snow on higher ground and the Dublin mountains is likely to experience snow," said Ms Ryan.
"There could be snow in other parts of the country and certainly it is cold enough."
Thermals should be at the ready with extreme cold a certainty over the next few days and temperatures are expected to reach a low by Wednesday.
"Those temperatures are well below normal for this time of the year," said Ms Ryan.
"It looks like Wednesday night is going to be the coldest night with severe frost.
"Day-time temperatures will be no more than three or four degrees and there will be scattered outbreaks of rain, sleet and possibly snow."
While frost and ice will see night-time temperatures plummet, day-time temperatures are also unseasonably frosty and not expected to rise above five degrees.
"Definitely it will be very cold and there will be a very brisk north-west wind with the wind- chill effect there.
"Temperatures will reach no more than four or five degrees today, but technically it will feel colder with that wind-chill effect."
Night-time temperatures will drop to around minus three degrees in Dublin. "There is a cold one to come tonight with temperatures dropping back to sub-zero right across the greater Dublin region.
"Tomorrow it's going to be frosty and icy and there's a risk of a wintry mix of rain, sleet and maybe snow," said Ms Ryan.
Despite the cold snap, Ireland will escape the weather due to hit Britain over the next few days, where a severe weather warning has been issued.