The book, of about 20 pages, has been dated to AD800-1000 and is the first discovery of an Irish early medieval document in two centuries, according to Trinity College manuscripts expert Bernard Meehan.
Never before has such a document been discovered buried in the soggy earth of Ireland.
"This is really a miracle find," said Pat Wallace, director of the National Museum of Ireland, which has the book stored in refrigeration and facing years of painstaking analysis before it is put on public display.
"There's two sets of odds that make this discovery really way out," Mr Wallace said. "First of all, it's unlikely that something this fragile could survive buried in a bog at all, and then for it to be unearthed and spotted before it was destroyed is incalculably more amazing."
He said an engineer was digging up bogland last week to create commercial potting soil somewhere in Ireland's midlands - he would not specify where because a team of archeologists was exploring the site - when, "just beyond the bucket of his bulldozer, he spotted something".
"The owner of the bog has had dealings with us in the past and is very much in favour of archaeological discovery and reporting it," Mr Wallace said.
Crucially, he said, the bog owner covered up the book with damp soil. Had it been left exposed overnight, "it could have dried out and just vanished, blown away".
The book was found open to a page describing, in black-letter Latin script, Psalm 83, in which God hears complaints of other nations' attempts to wipe out the name of Israel.
Mr Wallace said several National Museum and Trinity College experts spent Tuesday analysing only that page - the number of letters per line, lines per page, size of page - and the book's binding and cover, which he described as "leather vellum, very thick wallet in appearance".
It could take months of study, he said, just to identify the safest way to pry open the pages without damaging or destroying them.
"That is certainly going to be the nightmare, trying to separate the pages," he said, ruling out the prospect of using X-rays to investigate without physically moving the pages.
Ireland already has several other holy books from the early medieval period, most famously the ornately illustrated Book of Kells, which has been on display at Trinity College in Dublin since the 19th century.
