Aye, I don't think Shamanism reached as far south as the Celtic nations. It was fairly rare in Scandinavia, even, much more common among the Saami.
That statement is highly dependent on how you define shamanism which has been poorly defined so far. I am not fond of the term but it has persisted in research as a term for the representation of religious rituals and patterns dealing with the non-human world. An example of this definition is in
The Realities of Witchcraft and Popular Magic in Early Modern Europe: Culture, Cognition and Everyday Life, Palgrave Macmillan, Hampshire, 2008: 201-202, 211 by Edward
Bever
Gives us this perspective
“Shaman” can be reserved for a person who engages in practices involving the alteration of consciousness to perceive and interact with spirits to gain knowledge or power inaccessible in normal waking consciousness,
“shamanic” used to refer to such practices. In contrast, “Shamanist” can be used for someone who engages in other deliberate manipulations of consciousness to access unconscious information and skills not ordinarily accessible in waking consciousness, practices that do involve deliberately manipulating consciousness, their own or someone else’s, but do not generate the experience of perceiving spirits
“shamanistic” can be used to refer to such manipulations. Since both variants of shamanism involve deliberate actions, they can be distinguished as “shamanic practices” or “shamanistic practices,” with the difference being whether they involve perception of spirits...”
In the following article Klein evaluates shamanism in Norse religion.
"Seiðr & Shamans: Defining the Myth of Ritual Specialists in pre-Christian Scandinavia" by Sebastian Klein
He gives this perspective of shamanism
“…traits of a shaman. Often, a shaman is responsible for engaging with forces deemed responsible for causing problems – when relationships are scuttled, health is threatened, food is low or war is on the horizon a shaman steps up to “reestablish an orderly, healthy, and viable cosmos.” 10 There are many ways the shaman can achieve this, such as performing vision quests, seeking spirits, prophesy, sorcery and magic, and by magic I will borrow the definition provided by anthropologists Victor Turner and Pascal Boyer as a “trans human controlling power that can either be personal or impersonal.”
He gives several examples
1. ...law code of Magnús Hákonarson forbids the practice of magic use
“And these things belong to the wrong pagan belief: Charms and magic and what some call troll-riding, soothsaying and the belief in spirits of the land that dwell in the hills, barrows or lakes. So is (forbidden) to sit and predict the fates; and those who renounce God and the holy church as to find, in a barrow or in other ways, power and wisdom; as to those who try to raise the dead and those who dwell within barrows.”"
2. "The Icelandic family sagas (
Islendingasögur) are so rife with references to the various seeresses, prophets and sorcerers, many of whom exhibit remarkable traits that could easily be interpreted as shamanic, and much of the archeological remains within graves and barrows that have been found have also been interpreted as belonging to shamanic ritual specialists."
3. He sits Odin in particular as a god exhibiting Shamanistic practices.
In this respect Celtic religion also shows the same shamanistic patterns very closely resembling Norse religion. To me this is not surprising at all given the significant overlap of theses adjacent cultures.
