An interesting project to be sure!
I can see various possibilities, based on constrains and design choices.
There are faces suitable for body text with more or less subtle references to celtic/uncial/insular styles.
The questions I think one should answer in order to narrow the search could be:
- There need to be just one typeface, or there could be two (one more calligraphic for headers and such, and a "tamed" one for texts)?
- How long would texts be (longer text = tamer typeface)?
- There will be need of emphasis inside the texts? And, if so, it should be expressed with italics or it could be other means (darker weight, colour, a different style)?
- As always, expected language coverage should be taken into account.

A couple of typefaces that could fit the bill, devoid of any historical or stylistic accuracy (found looking for "uncial text"):
http://www.myfonts.com/fonts/eurotypo/farfa/
http://www.myfonts.com/fonts/delve/rieven-uncial-pro/

For a more modern take, SIAS' Hibernica could be an interesting choice. Perhaps paired with its elder sister Lapidaria for a wider range of styles and permutations:
http://www.myfonts.com/fonts/andreas-stotzner/hibernica/
http://www.myfonts.com/fonts/andreas-stotzner/lapidaria/

Farfa looks like a reasonable choice for body copy.

If, however, by "legible", one means a conventional body type, with only a nod in the direction of keeping the same feel, maybe a Jenson of some sort is what is sought. Centaur, for example, might be considered.

A conventional roman that just nods a little bit in the direction of the calligraphic... I can't think of one offhand. That is, I can't think of one that nods just a little bit more than Hermann Zapf's Palatino.

Or a calligraphic typeface that is still very close to a conventional Roman - also something I've been looking for, and have not encountered.

There is Goudy Thirty, but while I'm definitely not going to recommend that, there could be something like that in Goudy's œuvre.

Looking for one, I came across "Goudy Newstyle", but that was apparently made for a Shaw Alphabet-style project, although it looks like a conventional typeface.

(Hey, looking at the p and q, and to a lesser extent, the b and d, this could be pitched as a font for dyslexics!)

Amazingly enough, it is available in digital form - or it was; the links I've turned up to fonts by Red Rooster and Richard Beatty are coming up dry.

However, I was able to find Jenson Eusebius New Style, which I think is even better.

Although it has no specific Irish heritage, Beorcana might fit the bill:
http://www.beorcana.com/

Yes, I think that Beorcana is an excellent choice.

And then there's the Google Font Asul.

It’s OK to use a newer face for body.
Back in the day, the principle was to use progressively newer lettering styles as one proceeded down the hierarchy of type size, on the manuscript page.

I recommend two of my neo-incunabula styles, Paradigm and Goodchild.
Paradigm, loosely inspired by Ratdolt’s work, has a kind of built-in distress parsing the effect of letterpress, or looking like it was done with a calligraphic pen at the small size limit of its ability.
Goodchild is a robust Venetian, a Jenson revival, with a large x-height.
So both styles will hold up very small.

(For a fuller-featured Goodchild, please contact me directly.)

I’m not sure if this is relevant here, but, for Irishness, there’s still the Irish versions of Andron which yet await to see the light of a release …
The concept is similar to Lapidaria/Hibernica, but done in a rather classical serif style.

Have a look at this page on the Sumner Stone's site:
http://www.stonetypefoundry.com/magmasuperfamily.html#TrajanGaramond

That's brilliant guys, many thanks. I'm going to need a bit of time to look through all this!