The 320-year-old manuscript had been lying in storage in the civic building for almost a century without anyone realising its significance.
The dust was finally blown off the weighty tome when it and other artefacts were moved during the recent £11 million re-fit of the hall.
Historians could not believe their eyes when they opened the front cover of the Paymaster General’s book to find a detailed record of every soldier and regiment in the 35,000-strong army that accompanied Protestant William III to Ireland to do battle with deposed Catholic English monarch King James II.
Every payment made to troops during that period was noted by Paymaster General Thomas Coningsby in the log.
The Dutch-born king would go on to defeat James on the fields around the River Boyne in Meath — a victory that is still commemorated by Protestant Orangemen in Northern Ireland and further afield every July 12.
Belfast City Council has now presented Coningsby’s book to the Grand Orange Lodge of Ireland, which plans to display it at its headquarters in Schomberg House in east Belfast.
Dr Jonathan Mattison, a researcher with the Orange Order, said the book was a gold mine of information.
“This is a remarkable piece of history,” he said.
“It records the accounts of every regiment in William’s forces — there were Dutch, English and Scotch regiments and also Irish soldiers who joined the Williamite cause.”
He said the manuscript would have pride of place in a planned Orange Order interpretative centre. “We are extremely grateful to Belfast City Council for placing such an item with us,” he added.
Belfast councillor and Orangeman William Humphrey explained how the book had come to be stored at the City Hall.
“We knew that it had been presented to the old Belfast Corporation way back in the mists of time,” he said.
“But, we did not really appreciate just how much information there was in it, until we gave it a more detailed examination.
“You could almost say this was a narrative of the events of 1690 and the start of the Williamite Wars. It is incredibly interesting and fortunately has been maintained in pristine condition.
“We decided to give it on permanent loan to the Orange Order so that more people would have access to it.”