How to receive a visionary dream according to pagan sources

https://youtu.be/NtSsTxYRXMU

The video specifically looks at an irish rite known as Imbas forosnai performed by elite seer poets known as Filíd, also  the tairbfheis, a rite to determine the High king at the Hill of Tara. In Wales there were the awenyddion and in Scotland they had a pagan rite of prophecy called Taghairm. I also look at several Anglo-Saxon and Norse Icelandic saga sources discussing Ulfhednar, Hammramr, Elves, haunted barrows and seers and compare them with the dreams described by Homer and Pausanias in Ancient Greece. 

Sources:  

Chadwick, N., ‘Dreams in Early European Literature’, in: Carney, James, and David Greene (eds), Celtic studies: essays in memory of Angus Matheson 1912–1962, London: Routledge, 1968. 33–50. 

Martin Martin A Description of the Western Islands of Scotland (1703)   

O Rahilly, T. F., ‘Early irish history and mythology’ (Dublin 1946)  

Ramos, Eduardo, ‘The Dreams of a Bear: Animal Traditions in the Old Norse-Icelandic Context’ (2014) https://www.academia.edu/8461445/The_Dreams_of_a_Bear_Animal_Traditions_in_the_Old_Norse-Icelandic_Context  

Tendulkar, S. and Dwivedi, R., “Swapna’ in the Indian classics: Mythology or science?” (2010) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3215360/  

Vaschide and H. Piéron, ‘PROPHETIC DREAMS IN GREEK AND ROMAN ANTIQUITY’ (Oxford : 1901) https://archive.org/stream/jstor-27899218/27899218_djvu.txt   

The Wooing of Emer by Cú Chulainn (Author: [unknown]), p.303 (paragraph 78.) https://celt.ucc.ie/published/T301021/note038.html