The day wore on till it was old
she heard the bells that slowly tolled.
"Good folk, why do they mourning make?
In tower I hear the slow bells shake,
and Dirige the white priests sing
Whom to the churchyard do they bring?"
The key is in the reference in line 429 to "white priests". If we assume that this is a reference to the Cistercians (founded 1098), who were traditionally known as "the White Monks", then it means the action of the poem can't be taking place before circa 1100. The Cistercians underwent a boom of expansion in the early decades of the twelfth century under the missionary zeal of Bernard of Clairvaux; I haven't come across when they penetrated into Britanny, but in any case the evidence, such as it is, suggests the action might be taking place in the twelfth century — just in time to inspire "The Lay of Aotrou and Itroun", which takes the form of a thirteenth century Breton Lay.
Maybe, maybe not; just thought I'd share.