Some 28 sheep were killed or are missing after one attack in the Enfield area of Co Meath in the past week. A further 40 sheep were badly mauled.
It is believed the missing sheep drowned in the nearby River Boyne as they fled from the dogs.
There have also been reports of other sheep kills in flocks that are extremely vulnerable at this time of the year.
IFA sheep committee chairman Laurence Fallon said there are up to four million heavily pregnant ewes on 35,000 sheep farms across the country, all approaching a critical lambing period.
“A dog attack on a sheep flock at this time can inflict immense damage in terms of both animal welfare and financial consequence,” he said.
Mr Fallon said all dog owners must ensure that their pets are under full control at all times, and are not allowed roam freely in the countryside where they can inflict horrific damage on a sheep flock.
Owners should also be aware that, under the law, they can be held liable for the financial damage inflicted by an attack by their dog, he said.
ICSA sheep committee chairman Francis Cusack agreed, warning that unwanted dogs being unleashed into the countryside could cause havoc during lambing season.
He said there is a seasonal problem of unwanted dogs being dropped off in the country rather than being disposed of correctly.
This puts extra pressure on the resources of dog wardens and the ISPCA and leads to hungry, near feral dogs attacking sheep during the lambing season, he said.