A detonated explosive device was found near Newry last night.
The exploded device was found after British army bomb disposal experts were called to examine a suspicious object on the Barracric Road in Cloughogue.
And there was a second security alert in north Belfast last night after a suspicious object was found close to Ardoyne shops.
No damage was caused in the Cloughogue alert and there were no reports of any injuries.
The remnants of the device were taken away for further examination.
The railway line was closed between Newry and Dundalk for several hours yesterday evening while parts of the exploded device were examined.
Inspector Lorraine Dobson appealed to anyone who noticed any activity on the Barracric Road in recent days to contact officers.
"I would like to thank the public for their cooperation during this operation," she said.
Sinn Féin's Mickey Brady said the device had put people's lives at risk.
"The people carrying out these attacks are doing so against the expressed wishes of the Irish people and to wreck the advances of the peace and political process," he said.
"It not only was an attack on the political process but also potentially endangered people living in the area."
Police remained at the scene of the alert on Ardoyne Road in north Belfast last night.
A suspicious object was found close to Ardoyne shops at around 6pm. It is understood it was discovered by members of the public.
Ardoyne Road was closed between Crumlin Road and Alliance Avenue.
Estoril Park was also closed and several homes in the area and shops were evacuated.
It was unclear last night whether the suspicious object was a viable device, but the army bomb disposal robot was seen in the area.
SDLP North Belfast assembly member Alban Maginness said "all right-thinking people" should condemn the alert.
"I condemn this as yet another alert in the Ardoyne area caused by people who are intent in disrupting the peace in the community," he said.
"They are putting the lives of the public in danger. They are undermining public order and are intent on attacking or disrupting the police."
The alerts on polling day in the general election came as police set up checkpoints and extra patrols following dissident republican attacks in Belfast and Derry over the last fortnight.
Dissidents have been blamed for several attacks in the last few weeks, including two bombs planted at a Territorial Army base in Derry on Sunday and an attempted bomb attack on police in north Belfast last week, just metres from the scene of last night's alert.
The dissident republican group calling itself the IRA claimed responsibility for the attempted attack on police, a separate blast bomb attack on police in north Belfast last month, and an attempted ‘come-on’ bomb attack in Derry last month.
