I try to find: Gaeilge version of the liturgy of the mass by Sean O’Riarda. My irish friend told me that it possible exist in version written for pipes and Harpsichord. Thanks!
Beir bua is beannacht!
Did you consider contacting Peadar O’Riada?
POR’s website
I got my copy from Seolta:
http://www.seoltamusic.com/details.php?prodId=228&PHPSESSID=d11c39440ca21cf055b0f262fd3a64de
However, it was composed for choir and church organ and the sheet music is in Bb. No big deal if you have notation software which will allow you to transpose. However, the setting for church organ may not suit other accompanying instruments (such as harpsichord).