The discovery of the Psalter, or Book of Psalms, in the south Midlands is said to be one of the most significant discoveries in archaeology for decades.
National Museum of Ireland specialists believe the manuscript may have been lost in transit or dumped after a raid - possibly up to 1,200 years ago.
Museum director Dr Pat Wallace said it was "remarkably well preserved".
"Nobody has found anything like this for centuries - we are going to find it very hard to find people who know about it," he said.
The manuscript was found by a eagle-eyed digger-driver, who acted quickly to ensure its preservation.
Dr Wallace said it was not in pristine condition but some of the writing was still legible.
"When we saw it in the bog, we were able to read one of the psalms in Latin," he said.
He said it was "not so much the fragments themselves, but what they represent, that is of such staggering importance".
"In my wildest hopes, I could only have dreamed of a discovery as fragile and rare as this.
"It testifies to the incredible richness of the Early Christian civilisation of this island and to the greatness of ancient Ireland."
It is believed the extensive fragments are of an Irish Early Christian Psalter, written on vellum.
They were recovered from the bog by a bulldozer last week, and transported to the National Museum's conservation laboratory for painstaking analysis.
Trinity College Dublin head of manuscripts Dr Bernard Meehan said it was the first discovery of an Irish Early Medieval manuscript in two centuries.
"Initial impressions place the composition date of the manuscript at about 800AD - but how soon after this date it was lost we may never know," he said.
Once the manuscript has been conserved, it will be displayed in the Early Christian gallery of the museum, alongside the Ardagh chalice and the Derrynaflan paten.
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