Online trading platform Intrade has ceased all trading activity pending an investigation.
In a statement on its website it blamed the halt on "circumstances recently discovered... that may include financial irregularities".
The move means users will not be able to withdraw money from their accounts.
Dublin-based Intrade allows users to place bets on future events by buying and selling contracts in a similar way to the financial markets.
Users can bet on everything from who will be the next Pope to how much snow will fall in a given winter.
It is favoured by academics and journalists who often use the site's odds as indications of public sentiment, particularly when it comes to predicting US presidential elections.
"With sincere regret we must inform you that due to circumstances recently discovered we must immediately cease trading activity," Intrade said in its statement, saying the shutdown had been required under Irish law.
"These circumstances require immediate further investigation, and may include financial irregularities.
"During the upcoming weeks, we will investigate these circumstances further and determine the necessary course of action."
The company did not elaborate, and it is not clear if or when it will resume operations.
In November Intrade suffered a severe blow to its business when regulators forced it to suspend all trading in the United States - its biggest market.
The US Commodity Futures Trading Commission (CFTC) filed a lawsuit against the company, arguing that it violated US trading laws.
It lost tens of thousands of users as a result of the legal action.
Intrade was founded in 1999 by John Delaney. He died in May 2011 while attempting to climb Mount Everest.