This reticence, linked to Ireland’s painful experience of a banking crash in 2008 and subsequent international bailout, means Dublin is unlikely to become a major destination for what is regarded as some of the banks’ riskiest business.

Ireland’s central bank has indicated in talks with the banks that they would face high hurdles to win regulatory approval for such operations, which involve huge sums when compared with the relatively small size of the country’s economy.

“Ireland is being very realistic about what it can and what it wants to do,” said one source at a large global investment bank, speaking on condition of anonymity as the discussions are private. “If you’ve come from all the troubles Ireland has, you want to be very careful about taking on risks.”

The largely U.S., British and Swiss investment banks are working out how to secure access to the European Union when Britain leaves the bloc. The main question is where to trade and clear European securities, euros and other market activities controlled by EU regulation.

Such trade carries a lot of risk and large balance sheets, meaning regulators must supervise the banks’ trading models closely. This, along with the scale of the business, has prompted the cautious response from Dublin, according to the sources.

A spokeswoman for the Irish central bank said there was no blanket policy of turning certain types of business away. “The central bank is open to engagement with any firm wishing to obtain an authorization,” she said.

However, another banking source said Dublin had specific types of financial business in mind. “Yes, Ireland want insurers, asset managers, back office functions … but they don’t want big balance sheet risk. They just don’t want to take on that kind of risk and feel that they don’t have the regulatory bandwidth to do that,” the source said.

Reuters asked the five large U.S. banks as well as Barclays (BCS) and Credit Suisse (CSGKF), who have some operations in Ireland, whether Dublin was still a contender. All of the banks declined to comment.

Historic Opportunity

Until now, global banks have always put the bulk of their European markets businesses in London, which is by far the largest financial center in the EU.

When Britain leaves the EU financial firms based there are likely to lose their “passporting” rights, an EU system that lets them operate across the bloc but under the supervision of just one member state’s regulators.

That’s prompted the likes of Dublin, Paris, Amsterdam, Luxembourg and Frankfurt to encourage banks, insurers and fund managers to set up entities in their cities that can get licenses to operate across the EU. Ireland is also presenting itself as the only English-speaking country that offers a base in the euro zone and a future in the EU.

Kieran Donoghue, who heads up International Financial Services at IDA Ireland, the state agency charged with attracting foreign investment, has described Brexit as a “historic opportunity” for the financial sector.

Ireland is already one of the world’s largest centers for back office banking functions such as settling transactions, many of them farmed out from London. On top of that, it hosts a growing financial technology industry.

But it has a population of less than five million and its annual economic output is only around 10 percent of neighboring Britain’s. That left it vulnerable when disaster struck in 2008.

Irish taxpayers had to stump up 64 billion euros (now $68 billion)—or almost 40% of GDP—to rescue a banking system brought down by a property market crash.

The cost of staging the biggest public rescue of banks in the euro zone forced the state to take the 85 billion euro bailout in 2010. Conditions of the three-year EU/IMF program included deeply unpopular austerity polices.

A Real Worry

Investment banks with large sales and trading operations, which buy and sell foreign exchange, debt, equities and other financial instruments for clients across Europe, require large balance sheets, specialized talent and regulators who are familiar with sometimes esoteric financial instruments.

“Our sense is that the appetite in Ireland is not that high for balance sheet banks,” said a third source at a global investment bank.

Irish central bank governor Philip Lane told Reuters in October that his office had seen a jump in inquiries from financial services companies since Britons voted to leave the EU in June. However, he doubted activity will cluster in a single euro zone city because none offers a close substitute to London.

Central bank officials have also publicly said the authorization process for financial services firms wanting to set up in Ireland cannot be short circuited, and that board and management positions would need to be located in the country.

“A lack of specialized supervisors and the risk of sophisticated investment banking to the state makes Irish regulators reluctant to host such banks in Dublin,” said a person familiar with Irish central bank thinking.

“It has been a worry for a while. It is difficult to find enough regulators. A growth in highly sophisticated financial services companies would be a real worry.”

Under EU “bail-in” rules introduced since the global crisis, investors and uninsured depositors will be have to fund any future bank rescues rather than governments.

Nevertheless, the source still said the “risk to the taxpayer” was a second reason for concern.

The European Central Bank now oversees the largest banks operating in the euro zone, rather than member states, but it still relies on staff from national supervisory authorities to help carry out its work.

Bank executives say that as well as meeting the requirements set by Irish regulators, they would have to satisfy their home-country authorities that any operations they sited in Ireland were adequately capitalized and supervised.

“It’s not just down to the Irish regulators. It’s down to the British, and the U.S. and the continental regulators,” said another banking source.

Lengthy Approval

This year Credit Suisse became the first global investment bank to set up a trading floor in Dublin, with around 100 jobs offering prime brokerage services. About 40 are trading positions, with the rest in support functions.

That process for a very specific and narrow trading license took between three and four years from initial planning stages to approval, according to a source familiar with the process.

Banks would want to be able to move other, larger types of trading operations much faster, given Britain is expected to leave the EU in 2019.

U.S. bank Citigroup (C) has denied a report it was planning to move up to 900 jobs from London to Dublin as a result of Brexit, though it already has a large unit there providing some banking services. But like other U.S. banks, it would need to host large trading operations in a separate entity known as a broker-dealer under American regulations.