"I gave her gifts of the mind
I gave her the secret sign
That's known to the artists
Who have known the true gods of sound and stone
And word and tint, I did not stint,
I gave her poems to say.
With her own name there and her own dark hair
Like clouds over fields of May."

    This may be a bit risqué but in the old Celtic mysteries "gifts of the mind" and "the secret sign" are clear and common references to passing on one's knowledge.
    First one must give "the gifts of the mind" which allows the other to perceive things in the correct manner. Then one must give them "the secret sign"; this is a bit harder to describe but simplified can be likened to a spiritual "tattoo" or marking which acts as a kind of "Magician card" allowing one access to both the Otherworld and the confidence of others of like mind.
    Practitioners of the arcane were often called bards or artists and all artistic achievement was considered to be an extension of the same root ability.   The "true gods (note plural) of sound and stone" seems a clear reference to the Irish deities predating the Christian invasion.
    Working hypothetically and using the assumption that he's trying to initiate this girl the rest of the verse is also fairly blunt if one possesses a basic understanding of the practice. "Word and tint" as well as giving her poems to say refers to the practice of reciting poems in a certain manner, an extremely common means of casting a spell, something like the mantras of Hinduism. (This can be seen in almost every story in Irish/Celtic mythology.)
    "With her own name there and her own dark hair" is a reference to an initiatory experience. "Like clouds over fields of May" is a bit more vague but refers to a failed initiation or something going horribly wrong in the process. Clouds are a common metaphor for the haze of illusion that hangs over human existence which is lifted during initiation. Something akin to the enlightenment of Hindu/Buddhist tradition but not quite.
    I would add that poetry and mysticism have been linked in all surviving Celtic traditions since mythic times. This can me seem in the likes of Taelsin (Welsh), Merlin (Welsh, Breton, Cornish), and Cuchalain (Irish) as well as many more modern poets and playwrights.
    I would add that this does not conflict with anything written previously, it was common for such knowledge to be passed from mate to mate and then on to their children. It seems to me the whole poem refers to a man encountering a woman he finds to me beautiful and attempting to bring her in to his world. This attempt fails due to her being made of "clay", or her not being capable of understanding such things.
    By attempting to drag her into his world for his sake and not hers he is doing "not as he should" and as a consequence she instead drags her into her world, hence he loses his wings. (Note the very clear indication of the angel as male.)
    Whatever the meaning it is indeed a truly beautiful work and I mean no offense by this interpretation. I also agree totally agree with the statement about metaphors with multiple meanings. I offer this as yet another dimension of this masterpiece.