2Harbison, Peter. "How Old Is Gallarus Oratory? A Reappraisal of Its Role in Early Irish Architecture." Medieval Archaeology 14 (1970): 34.
In 1756 Charles Smith reported that "some think" Gallarus may have been constructed using an earthen mold: "...a heap of earth was first raised, in the form of the inside of the cell, and that they built over it, and wedged in the key-stones at the top, over which are a range of loose stones laid like a ridge; and the structure being thus finished, they carried out all the earth at the door; and lastly, smoothed the walls on the inside with chissels, &c." (Smith, Charles. The antient [sic] and present state of the county of Kerry: Being a natural, civil, ecclesiastical, historical, and topographical description thereof. Dublin: Printed for W. Wilson, 1756. 192.) This book may be read in its entirety here.