A wind farm formerly owned by Sean Quinn's family has been targeted by arsonists.
Police said an electrical substation at the property on Ballyconnell Road in Derrylin, Co Fermanagh, was set alight yesterday evening.
No one was injured but the fire caused substantial damage.
The wind farm is one of a number of businesses in the area once run by the Quinn family that were placed into administration after the tycoon went bust.
A PSNI spokeswoman said: "At this stage, police believe the fire was started deliberately."
Workers at Quinn Group factories in the area staged a one-day walkout on Monday following the jailing of Quinn last week.
Protesters used tractors and other agricultural vehicles to block the entrances of the cement and glass plants in Derrylin and just across the border in Ballyconnell, Co Cavan.
They called for immediate mediation between the former Anglo Irish Bank and the Quinn family over the long-running debt row that culminated with the imprisonment of the fallen tycoon last Friday.
Protesters have demanded Quinn's release while the talks take place
The 66-year-old was jailed in Dublin's High Court over a multimillion-euro asset-stripping plot.
He was sentenced to nine weeks for contempt of court for his part in putting the Quinn family's €500 million international property portfolio out of the reach of Anglo, now rebranded as Irish Bank Resolution Corporation (IBRC), and a subsequent failure to retrieve the money.
The IBRC claims the broke businessman owes it a total of €2.8 billion after running up unprecedented losses through secret stock investments in Anglo as its share price collapsed.
The family admit they owe €455 million but have refused the claims on the rest and have taken a counter-case against the IBRC over a loans deal.