Irish police are investigating the discovery of a bomb at a house in County Louth being built by the Ulster Unionist peer, Eddie Haughey.
The house is being built near Hackballscross
Army bomb experts were called to Lord Ballyedmond's property at Drumgooley and the device was made safe.
Gardai said it contained 70 pounds of homemade explosive mix packed into a gas cylinder and would have destroyed the house if it had gone off.
Security sources said dissident republicans were behind the attack.
The device, found by a farm labourer just after 1230 BST on Tuesday, had been packed into a cylinder and hidden in one of the walls of the house.
BBC Northern Ireland Dublin correspondent Shane Harrison said sources said the detonator had been set off, but it failed to explode the bomb.
"Sources also said they believed dissident republicans, who have been more active in the border area over the past week, are responsible," he said.
Northern Ireland Office Minister David Hanson said it was "a cowardly attack on someone who has worked tirelessly to create jobs and bring investment into Northern Ireland".
"His efforts to promote Northern Ireland on an international stage contrast sharply with those who left this device at his home and who want to drag us back to the past. Their actions will not succeed," said the minister.
Ulster Unionist deputy leader Danny Kennedy said the incident was "extremely worrying".
"This is attempted murder apparently carried out by republican dissidents in the north Louth area," he said.
The device was found at Lord Ballyedmond's house
It is believed Lord Ballyedmond was building the house, which is near Hackballscross, to replace a previous dwelling which had been on the site since the time of his grandfather.
Lord Ballyedmond, who set up Norbrook Laboratories, is a former senator in the Irish Republic.
The device has been taken away for further examination.