By Clodagh Kilcoyne

KILDARE, Ireland, July 26 (Reuters) - Irish racehorse trainers and breeders fear a Brexit that impedes the free movement of animals could threaten its success on the track and position as Europe's largest producer of thoroughbreds.

Ireland is widely considered the European Union member most at risk from next door Britain's exit from the bloc, potentially cutting its land route to mainland Europe for exports and hindering trade with its key UK market.

Few sectors are as integrated as the two islands' racing and breeding industries that, according to Horse Racing Ireland, the national authority for thoroughbred racing, contribute over 1 billion euros a year to Ireland's 189-billion-euro economy.

"Our fears are that if there any trade barriers or any tax or tariffs on movement of horses between Ireland and England, that could have a detrimental effect," said Henry Beeby, chief Executive of Goffs, Ireland's leading bloodstock sales company.

"With bloodstock, we are an export nation and rely heavily on being able to do that unencumbered by any restriction," Beeby said, referring to the two-thirds of foals born in Ireland each year that are exported, 80 percent of them to Britain.

For Ireland's world-renowned trainers, the stakes are just as high.

About 10,000 racehorses travel between Britain and Ireland each year. The proximity and ease of access to the home of so many of the industry's major events plays a central role in Ireland's successful track record, Horse Racing Ireland say.

Of the 28 winners at March's showpiece Cheltenham festival, a record 19 were Irish-trained, including Gold Cup victor Sizing John.

Irish-trained Rule The World was the surprise 33-1 winner at the 2016 Aintree Grand National. And at last year's Royal Ascot festival, one in three races were won by Irish-trained horses and almost two in three winners were foaled in Ireland.

"In effect, we are twin industries, joined at the hip," Horse Racing Ireland chief executive Brian Kavanagh told an Irish parliamentary committee on Brexit last month.

"Unlike many other Brexit-hit sectors, we simply cannot adapt our product to suit new markets. Royal Ascot, Cheltenham, Aintree and Epsom cannot be replicated in another country."

Ireland, France and Britain - three of Europe's top horse racing locations - currently have an agreement that predates EU law allowing a horse registered in one country to move freely in all without the need for veterinary examination or inspection.

That includes the border between the Irish Republic, an EU member, and Northern Ireland, a British province, where nine out of 10 horses that compete are trained in the republic.

Jessica Harrington, trainer of champions including Sizing John, recalls when the border was marked by checkpoints before a 1998 peace deal ended Northern Ireland's sectarian conflict. Racehorses, highly sensitive animals bred for their flight response, would sometimes be stuck in boxes at the frontier for hours.

"If we are going to go back to what it was, it's madness," Harrington told Reuters from her yard at Commonstown stables in County Kildare.

She also fretted for the British landbridge trainers who have traditionally transported their horses to mainland Europe after an overland drive through Britain to save them a lengthy boat journey direct from Ireland.

"Are they going to say that you have to have a sealed horse box? Has anyone thought about these things? Horses can't do that, you can't do that. By law they are only allowed to do so many hours and then they have to rest," she said.

"We've talked about it in the trainers association and nobody knows. It's now damage limitation more than anything else...No one has a plan."

Click here to see The Wider Image: http://reut.rs/2h0RvX9 (Writing by Padraic Halpin; editing by Raya Jalabi/Mark Heinrich)