Very little in the ancient Celtic mythos is truly about Spirit, so we have to go back into Hindu writings to find some ideas that the Celts might have shared. The Isha Upanishad describes Spirit this way:
Although not moving, the self is swifter than the mind;
The gods cannot catch It as It speeds on in front.
Standing, It outpaces others who run; within It Sages place the waters.
It moves–yet It does not move. It’s far away–yet It is near at hand!
It is within this whole world–yet It’s also outside this whole world.
Recall that the Hindus thought of Spirit or Life as being equated to breath, so that the Ultimate Deity is sometimes defined as the Living God that does not breathe, or the Breathless One.
A more modern way of looking at it is defined by philosopher Georg Hegel:
The nature of spirit may be understood by a glance at its direct opposite–Matter.
As the essence of matter is gravity, so, on the other hand,
we may affirm that the substance, the essence, of spirit is freedom.
If indeed it is true that every living thing has a spirit, then each cell of your body has a spirit. Is your spirit then the sum of all these minute spirits working in comjunction? Or is your spirit kind of a dictator-spirit that organizes and runs all the smaller entities?