I have also given up trying to find a Spiritual group, and I don't follow a Celtic path, per-se. However, to me, what you are asking goes back to an old, old discussion about the nature of "god-head", or Universal Power, (or insert any other name that you like or use).
Forgive me for digressing, but I don't know any other way to explain how I got to thinking about this.
I tried Unitarianism, and I am still quite fond of Unitarians, because so many of them are thoughtful people, who are well read, and who actually give thought to what comes out of their mouths.
Unfortunately, where I live, the local Unitarian meeting place has become so tolerant (maybe that is not the right description, but I can't think how else to word it), that, to me at least, it has become such an "anything goes" group, that any person with "odd ball" ideas uses it as a Forum to push their own agenda.
I learned something there though, when I took a class on the history of Unitarianism, that I had not been aware of during my long past association with the Unitarians.
I learned that Unitarians are the product of a school of thought that goes back to the beginning of Christianity.
Some say that the "Universal Power" source is 'ONE'.
The Nicene Council, which sought to unify the theology of the Christian religion in the year 336, mandated that the concept of a Trinitarian God be a cornerstone of the Doctrine.
Now, not everyone believed in this Trinitarian concept, including the Bishop who baptised the Roman Emperor Constantinople into Christianity.
Some say, that, since Christianity had been declared the "official" religion of the Roman Empire, the Romans wanted to make Christianity more attractive to the multitudes of Roman subjects who had been Pagans.
Paganism, in its myriad forms, often has a 'triune' type of god concept.
Examples, such as Maiden, Mother, Crone, or, Land, Sky, Sea, and other "three in one" concepts like this, can sort of be tied to the Trinity, in the 'form' of "Father, Son, and Holy Ghost".
In other words, creating a concept of the Trinity, in the official Roman religion of Christianity, made it easier for former 'pagans' to swallow, since they were used to "three in one" god concepts.
I still am sorting out my thoughts about this "Trinity vs Unitarian" concept of god, Universal source, Higher Power, (whatever), but I guess, since it is such an old discussion, that no one has been able to resolve, except in the privacy of their own hearts, minds, spirits (hey, that sounds trinitarian, now that I write it!), so I guess I am in good company.
I had known about the three worlds, but had not heard of Manannan Mac Lir; seeing how we've had several days of pea-soup fog, his mention is very timely!