"Along Comes Mary" is a 1966 song by pop music group The Association, written by Tandym Almer (who was not in the group). The song was commercially successful, reaching #7 on the Billboard charts.
It is hard to describe, in either technical or non-technical terms, what "psychedelic" means in terms of music. Many songs that have nothing in common musically feel "psychedelic". That being said, this song is psychedelic. The bass line growls while the rest of the music and vocals seems to twist and swirl around. The lyrics change from a staccato chant to a soaring chorus. They do suffer from being hard to understand, except for that chorus:
Along comes Mary...and my empty cup is as sweet as the punch!
As for what the lyrics are about---take your pick. A lonely man encounters either a girl named Mary, Marijuana, or The Virgin Mary. Coming out in 1966 with its "trippy", "groovy" feeling, it seems likely it could be a weed song--but then, of course, so can anything.
What is most interesting to me is how this song managed to become a victim of the chasm between "Rock and Roll" and "Rock Music". There was a short period---barely from 1965 to 1967, when the love songs with a good beat were turning into something more introspective and experimental, but that would soon be eclipsed by hard rock. I encountered this song on oldies radio, where it was an upbeat singalong for commuting moms, even though this song is much closer, in time and feeling, to Jimi Hendrix, than to Elvis Presley. Like much of the music from this short time period, it fell through the cracks of being taken seriously.