The only church in the Republic belonging to the Rev Ian Paisley's Free Presbyterian Church, in Co Monaghan, was damaged by fire early yesterday morning.
Gardai were alerted shortly after 5 a.m. that there was a fire in Coragarry church outside Drum, near Clones.
A spokesman said a Portakabin extension used for Sunday school classes had been destroyed, the front hallway damaged and there was smoke damage in the rest of the church.
He said the scene had been preserved for a Garda forensic team to examine, and it was too early to say whether it had been an arson attack.
If it turns out to have been arson, it will be the first such attack on the small Free Presbyterian church since it was opened by Dr Paisley in 1975.
The village of Drum is unusual in the Republic in that its inhabitants are overwhelmingly Protestant.
Meanwhile, Orangemen in Warrenpoint held their Somme anniversary parade service yesterday afternoon several hours after their hall, in Great George's Street, sustained extensive damage in an arson attack.
The hall is used as a meeting place for two Orange lodges, a Royal Black Preceptory and Warrenpoint Silver Band, but it is thought the building may have to be demolished.
The roof and upper floor were extensively damaged after arsonists made a hole in the roof.
Two SDLP councillors, Mr Jim McCart and Mr Hugh Carr, surveyed the scene yesterday and expressed their sadness at the attack. "We speak for the vast majority of the townspeople in expressing our revulsion at this attack on the unionist people of the area. We reject totally the warped mentality that produces such actions."
The incident was discovered by a passing police patrol around 4.30 a.m. on Sunday.
The Ulster Unionist Assembly member for the area, Mr Danny Kennedy, said the attack was despicable. He added that it was clearly a deliberate attempt to raise tensions within the community, and appealed for people to remain calm.
South Down SDLP MP, Mr Eddie McGrady, endorsed Mr Kennedy's appeal.
"This act is obviously done by persons who are intent at this very difficult time on stirring up community tension, hatred and violence," he said.
He added: "The perpetrators of this act display their total intolerance of other people's beliefs and practices."
Police are investigating a suspicious fire at a mission hall in Co Derry. The hall in Tobermore was destroyed in the incident, which was discovered around 7.30 a.m. on Sunday.
Meanwhile, a number of vehicles were set on fire in north Belfast on Saturday night. Police have appealed for anybody with any information to contact detectives in Belfast or Crimestoppers on 0800 555 111.
The DUP's constituency office in Ballymena was damaged in an arson attack at the weekend. The party leader, the Rev Ian Paisley, condemned the incident, describing it as a cowardly and despicable attack. "This office helps thousands of people of every religion. It was a cowardly attack not only against the DUP but against the entire community," he said.
The RUC confirmed that a lighted paper had been pushed through the door, but only minor damage was caused to the porch before the fire went out.