A leaf from The Book of Kells, an illuminated Gospel Book (c. 800 AD), roughly contemporary with our text of the Navigatio. Image © The Board of Trinity College Dublin, University of Dublin.
Navigatio Brendani 12.42:
Erat enim quadrāta, tam longitūdinis quam lātitūdinis
Most churches in real life were rectangular, but a square shape was thought of as an ideal reflection of the Temple in Jerusalem. In this page from the 9th century Book of Kells the Four Evangalists are in the corners. Christ is represented as an aniconic lozenge at the center of the CHI (X), and an integral part of the (almost) square Temple. See Tomás Ó Carragáin, Churches in Early Medieval Ireland (2010), pp. 40–41.