The "-tantiality" is just a suffix for "con-" and "trans-". The "con-" refers to consubstantiation, the belief in Catholic theology that the father, the son and the holy spirit are all of the same substance but each presents in a different form. The "-trans-" refers to transubstantiation, the belief in Catholic theology that during communion the bread and wine literally become the body and blood of christ; the mundane is transformed into the divine. These two first parts of the word indicate that the word has to do with the intersection between the divine (god) and the mundane (humanity), or christ. The "magnific" refers to the magnificent, the divine. The "-and-" simply refers to the word "and" and the "-jew-" refers to a Jew, specifically Mary. The "-bang-" is where things get tricky and we see a bit of Joyce's (slightly blasphemous) humour: in one sense, "-bang-" refers to a collision between the divine and Mary (christ). It also refers to sex; Mary is impregnated by the divine and out of this tryst christ is born. Joyce intentionally juxtaposes sexuality and the virgin because he both believes that all human activities are beautiful and deserve to be represented in literature and because, in order for christ to have a human mother and a divine father, he must have blood from both.
English Professor: You get an "A," and you don't even have to come to class anymore.
Catholic Priest: STOP READING JAMES JOYCE WE NEED TO BURN THAT FILTHY BOOK BEFORE IT POISONS MORE MINDS AND DAMNS MORE SOULS!!!
Hot Girl: What does contransmagnificandjewbangtantiality mean?
You: I better come over and explain it to you. We may even need to act it out...