Button mushrooms | Franck Fife/AFP via Getty Images Brexit kills Irish mushrooms The collapse of the pound is pummeling EU businesses dependent on exports to the UK.

TIPPERARY, Ireland — Ireland's farmers have become Brexit's first victims.

Seated at her farmhouse kitchen table, Lavinia Walsh frowned over a payment ledger that sums up the grim fate of the Irish mushroom industry. Since Britain’s vote to leave the European Union, the pound has sunk and is hauling down the livelihood of thousands of Irish farmers with it.

“The night before Brexit, I thought 'This will be fine, they’re not going to do it.' And they did. I couldn’t believe it,” said Walsh, 54, nursing a cup of tea on the farm where she lives with her two children and husband Donal. “It has developed into a catastrophe. It is a catastrophe for us.”

Five Irish mushroom farms have gone out of business since the June 23 vote. More are expected to follow by Christmas.

Like many Irish mushroom farmers, the Walshes sell their entire crop to the British market for prices fixed months in advance in sterling. The currency’s 15 percent drop against the euro since the Brexit referendum has wiped out their operating margin and tipped the sector into a crisis that could spell trouble for all exporters to the U.K.

"The mushroom producers were the first casualties of Brexit" — Pat Deering, MP

The Irish are not alone. Because the U.K. imports almost half of its food, the impact is widely felt.

Sterling's plunge is “affecting all the growers in Spain,” the U.K.’s biggest supplier of fruit and vegetables, according to Jorge Brotons of exporters association Fepex. The Netherlands, the next-largest supplier of vegetables to the U.K., has set up a dedicated Brexit information desk to help companies concerned about the fallout. A survey of British retailers by Barclays found that 43 percent expected to reduce the number of products they source from the rest of Europe.

But nowhere is as exposed as Ireland. The country is the U.K.’s single biggest supplier of food and drink, an industry that provides about one in 12 Irish jobs. The U.K. buys 84 percent of Ireland’s poultry exports, 65 percent of its cheddar cheese exports and 80 percent of the entire Irish mushroom crop.

"The mushroom producers were the first casualties of Brexit," said Pat Deering, a lawmaker in the ruling Fine Gael party.

In free fall

Donal McCarthy, head of mushroom producers' organization CMP, said that 10 percent of the Irish mushroom industry could disappear in the next few months.

“The industry is in free fall,” McCarthy said. “We need help and we need it now, because we're bleeding.”

Industry representatives last month sought government help to tide mushroom producers over until either sterling recovers or prices rise in Britain. But when the 2017 budget was announced last month, their proposals were not included. Deering suggested that the government could have been reluctant to set a precedent.

The size of the mushroom industry, which employs about 3,500 people and is worth roughly €180 million a year, may make it only a speck in the context of Ireland's €26 billion agri-food industry, but its fate is ominous for bigger sectors.

“It isn’t the mushroom industry alone. In beef there are huge challenges as well. Whatever we’re exporting to England is going to face difficulty,” Deering said.

A walk through the Walsh farm, 12 mushroom tunnels set in the deep green landscape of central Ireland, illustrates the mushroom sector’s particular vulnerability.

Farms ... were already operating on the tightest of margins due to a price war among British supermarkets.

To keep up a constant crop, Walsh must buy mushroom compost each week for €10,500, a price fixed in euros. She employs roughly 30 people — welcome jobs in rural Tipperary — and labor accounts for nearly half of her production costs. The mushrooms are on British supermarket shelves the day after they are picked. As mushrooms have a shelf life of just over a week, Walsh is largely a prisoner of her nearest market.

Farms like this were already operating on the tightest of margins due to a price war among British supermarkets, where incumbents such as Tesco are trying to stave off rise of German discounters such as Lidl. That cutthroat competition means retailers are highly reluctant to raise prices now, let alone renegotiate existing contracts that were agreed upon when one pound was worth €1.30, and not nearing parity.

Pastures new

The fear whispered among Irish mushroom farmers is that suppliers may turn to Poland, where the minimum wage is less than a third of Ireland’s and where producers blocked by sanctions from selling to Russia are looking for new markets.

While closures mean less competition, Walsh feels no pleasure in seeing mushroom farmers she knows personally go out of business. She fears the Irish mushroom industry may lose its bargaining power in the U.K. as a whole.

"What makes me angry, truly angry, is that they did nothing to deserve this. They were good growers," Walsh said. It is by far the toughest time she has seen since she and Donal set up the farm in 1999. “We can only hope that we’ll be there at the end of this. We intend to be here because of the age of our children. We need to get them through college. And that means digging in and carrying on.”

Simon McKeever, head of the Irish Exporters Association, said companies were thinking of moving people or production to the U.K. “It’s the most acute challenge that the Irish economy is facing. Aside from the 2008 downturn, this is the biggest challenge we have seen.”

Uncertainty is a large part of the problem. Exporters to the U.K. do not know whether their produce will be subject to tariffs in three years' time. Any eventual tariffs could “cripple” the Irish food industry, former Prime Minister Bertie Ahern told a U.K. parliament inquiry this week.

Communities living along the meandering border between Northern Ireland and Ireland — a divide that has caused decades of bloodshed — have been protesting the prospect of customs checks splitting the island.

Ultimately, the outcome is to a large extent out of Irish hands. In McKeever's words, Ireland is stuck between an irate Brussels and “a faction of the U.K. Conservative Party.”

“There is going to be a war of words and we are caught in the middle,” McKeever said. “It’s politics that are driving this, not economics.”

Walsh described going through every option available to cut costs. Delaying repairs. Extending credit lines. Keeping her cooler running on a "wing and a prayer." She pushed the payments' book away and sat back in her chair.

“What can I do? I can’t control the uncontrollable,” Walsh said.

She zigzagged her hand up and down to trace the volatile course of the sterling/euro exchange rate.

“It’s down to when Theresa May sneezes.”