Battle of the Boyne tour guides have been banned from wearing clothing that is either green or orange.
The County Meath site is where the Protestant King William III defeated the Catholic King James II and VII in 1690.
The victory of the Dutch King, whose family colour was orange, ushered in an era of Protestant domination across Ireland and Great Britain that would last for centuries.
As such, the Boyne is a hugely significant site for many unionists and the battle is marked each year in Northern Ireland with the 12th July bank holiday.

Given the historical sensitivities associated with the site, it has emerged that guides are asked not to wear green or orange - lest a guest infer some kind of political bias from them.
On Newstalk Breakfast, historian Ruth Dudley Edwards said her reaction was one of “absolute weariness” when she heard about the edict.
“I’m so sick of labels of all kinds,” she said.
“I’m so sick of the assumption that you go out dressed as your political views.
“I’m so sick of rainbow colours, for instance; I remember when Pride was one day, it’s now a month and it’s all over the place.
“I also have this vision of bureaucrats, well meaning, busy people, sitting at their desks saying, ‘Now what can we control next? What is the next possible problem? Oh yes, you might wear something green and upset an orange man because they’re touchy - or vice versa.’”
Battle of the Boyne tour
Ms Dudley Edwards, whose grandmother was in Cumann na mBan but is herself a unionist, continued that she would agree with a ban on clothing that contained any kind of political symbol.
“If Celtic t-shirts are being worn as a political weapon, well, it’s perfectly reasonable to tell the guides that’s really rather offensive,” she said.
“But otherwise, you can’t ban colours.”

Ms Dudley Edwards added that, in her experience, the vast majority of people do not care what colour clothing a person wears.
“A couple of years ago, I was asked by the Traditional Unionist Voice - which is the most hardline unionist party - to speak to them as an outsider about what they were doing wrong really,” she said.
“I wore my new suit, which was bright Kerry green because it happens to be my favourite colour, it always has been.
“I remember looking at it in the morning and thinking, ‘I should wear this - but should I wear this? Is it going to offend them.’
“And I thought, ‘Sod it, they’re grown ups’. I wore it and nobody said a word.”
The OPW has been contacted for comment.
Main image: A tour of the Boyne Battlefield. Picture by: Alamy.com.