New word! It describes a collection of texts (poetry and prose) from early Irish literature explaining the origins of place names.
From Oregon Quarterly article, Oregon’s Epic Estuaries, by Michael Strelow:
In twelfth-century Ireland, every aspiring poet had to learn—in addition to meters, forms, and techniques—dindsenchus, or the lore of high places, the topography of all the important places in Ireland about which a poet might write. Dindsenchus included prevailing winds and rains, prominent plants, limestone caverns and outcroppings, magical properties of the landscape, and especially water—loughs (lakes, including lakes that come and go seasonally), rivers, sea inlets, and springs. A poet could begin to write only after being certain of the where of the poem.
Knowledge of the real or putative history of local places formed an important part of the education of the elite in ancient Ireland. This formed part of the training of the military, for whom a knowledge of the landscape was essential. It was also essential knowledge for the bardic caste, who were expected to recite poems answering questions on place name origins as part of their professional duties. Consequently, the dindshenchas may well have grown by accretion from local texts compiled in schools as a way of teaching about places in their area.