Long after the mythological cycles were written down, the name Donn continued to surface in Irish folk tradition in ways that folklorists have puzzled over connecting to the god of Tech Duinn. In County Limerick, oral tradition collected from the nineteenth century onward describes Donn Firinne, a fairy lord said to dwell inside the hill of Cnoc Firinne and to control storms, thunder and the harvest, appearing sometimes riding a white horse before bad weather, a figure propitiated and blamed by local farming communities much as an old god of the dead and of the elements might be. Along stretches of the southern and western coast, sailors and fishing communities into the modern period were recorded attributing storms and drownings near the western headlands, the same stretch of sea associated with Tech Duinn, to the anger of "Donn," treating the old god less as a distant mythological figure than as a real, locally present danger to be respected.
Scholars of Irish folklore, T. F. O'Rahilly among the earliest, have argued that these later fairy-lord and weather-spirit figures named Donn are genuine survivals of the mythological ancestor-god of the dead, gradually reduced in status from a full deity of the Otherworld to a regional fairy king, though others caution that "Donn" was also simply a common personal and place name in Irish, so that not every later Donn figure need trace back to the god of the mythological texts. What is not in doubt is that, uniquely among the more obscure gods of the Tuatha De Danann's era, some version of Donn kept a living, feared presence in ordinary Irish belief for centuries after the manuscripts that first named him were written.