Previously, it had been thought that rain would be more likely - with perhaps sleet and snow on higher ground - and the Met Office was warning of the possibility of localised flooding.
But the alerts - including an amber warning, which is the second highest used by forecasters - now say a heavy band of sleet and snow will move in from the east throughout Friday and continue on Saturday, Sunday and into next week.
Forecasters say Friday will see "5-10cms of snow overnight into Saturday and as much as 15-20cm above 200m, with drifting".
The rest of the UK has already been under an amber alert amid the ongoing wintry conditions.
Blizzard conditions are expected across parts of England, Scotland and Wales and a red alert - the highest used by the Met Office - has been issued for Wales.
While temperatures in Northern Ireland are expected to remain in and around freezing, it's likely to feel significantly colder - more like -6C.
The public is warned to prepare for icy road conditions and possible weather-related disruption.