The approximately 20-page book has been dated to the years 800-1000. Trinity College manuscripts expert Bernard Meehan said it was the first discovery of an Irish early medieval document in two centuries.
"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 being put on public display.
"There's two sets of odds that make this discovery really way out. 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 bog-land last week to create commercial potting soil somewhere in Ireland's midlands when, "just beyond the bucket of his bulldozer, he spotted something." Wallace would not specify where the book was found because a team of archaeologists is still exploring the site.
"The owner of the bog has had dealings with us in past and is very much in favor of archaeological discovery and reporting it," Wallace said.
Crucially, he said, the bog owner covered up the book with damp soil. Had it been left exposed overnight, he said, "It could have dried out and just vanished, blown away."
The book was found open to a page describing, in Latin script, Psalm 83, in which God hears complaints of other nations' attempts to wipe out the name of Israel.
Wallace said several experts spent Tuesday analyzing only that page — the number of letters on each line, lines on each page, size of page — and the book's binding and cover, which he described as "leather velum, 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. He ruled out the use of X-rays to investigate without moving the pages.
Ireland already has several other holy books from the early medieval period, including the ornately illustrated Book of Kells, which has been on display at Trinity College in Dublin since the 19th century.