The Scottish man arrested by Garda Special Branch officers on Saturday in connection with an alleged threat to poison water supplies in Britain, has been arrested several times in the Republic and in Scotland and has been the subject of repeated investigations.
According to sources who know the man, he is highly eccentric and prone to sending threatening letters to government figures, but unlikely to pose a serious threat.
The man, who is in his 50s, arrived in Dublin about 10 years ago. He claimed he was fleeing harassment by British security services, who were investigating him for suspected involvement in a campaign of minor bomb attacks in Scotland associated with a group known as the Scottish Liberation Front.
He is suspected of sending letters two months ago to the British and Irish prime ministers and the Papal Nuncio to Ireland, Dr Luciano Storero, threatening to poison water supplies in Britain.
The letters described how this would be done and it is understood there were descriptions of adapting herbicides for use as poison.
The letters were signed on behalf of the Republican Revenge Group, which is believed to exist only in the man's imagination.
They said that there would be attacks on the water supply if Britain did not immediately withdraw from Northern Ireland. However, the letters contained sufficient detail of a threat to poison the water system to cause alarm.
Since the use of nerve gas to attack commuters in the Tokyo subway system some years ago by the Arum sect, police forces throughout the world have been on the alert for possible repeat attacks by extreme political or religious groups.
There are regular international exchanges of information among police and security services about threats from groups or isolated nations with the ability to manufacture highly dangerous toxins.
Manufacturing chemicals which can create highly toxic effects involves relatively low technology.
American police forces have gone to the extent of issuing anti-nerve gas agents to civil defence and police forces to be able to operate in the event of an attack on a city.
It is understood that concern about the letters sent last month to the two prime ministers and the Nuncio led to a joint Garda Special Branch and British Secret Service (MI5) operation to monitor the main suspect.
The man, who has lived mainly in west Dublin and who has given interviews with some newspapers in recent years, has a record of sending threatening letters.
He was convicted at Dublin Special Criminal Court in 1997 of sending bomb threats to newspaper and news agency offices two years earlier.
He is formerly a well-known Scottish nationalist who was suspected of being one of the founders of the Scottish Nationalist Front.
This group carried out some small bomb attacks on public utilities, including water pipes, in the mid-1980s.
It since emerged that the group was finally infiltrated by Scottish Special Branch and MI5 who were interested to know if it had any connections with Irish or other foreign terrorist groups.
The Scottish group gradually broke up and one member was killed - believed to be as a result of an internal feud.
After the man came to Dublin he made contact with splinter republican groups including the Irish National Liberation Army (INLA).
Republicans, however, regarded him as suspect and he was never taken into the confidence of any republican terrorist groups.
According to sources who know the man, he may have continued writing threatening letters to draw attention to his grievance that he was being pursued by the British security services because of his activities in Scotland in the 1980s. However, there have been no warrants for his extradition and he is not suspected of ever being a serious terrorist.