Just for the record, in traditional Latin, “missa” is not from “missio.” The ablative of “missio” would be “missione.”
However, in medieval Latin, words ending in “-io” are sometimes misspelled to end in “-a” (actually, it is rather anachronistic to call it “misspelling,” since they had nothing like Webster’s Dictionary at that time to use for standardizing spelling). Thus, assuming the Catholic Encyclopedia’s interpretation is correct, “missa” would be an alternative spelling for “missio,” which would be nominative, not ablative. Thus, “missa est” would mean “it is the sending forth.”
In a more usual interpretation of the Latin, “missa” appears to be part of a verb. “missa est” would be the complete verb–to be specific, the third person active perfect indicative form of the verb “mitto, mittere,” which means “to send” (or perhaps in this case, “to dismiss”). The form “missa est” means literally “she was sent.” “missa” in this case would be a feminine form representing an unnamed subject. Who is “she”? Well, in Latin, many things that we would refer to as “it” are referred to as feminine, for example, in modern English if we were to refer to sending a ship to war or something we would say, “it was sent,” but the Latin equivalent (“navis missa est”) would be abbreviated “missa est,” which literally means “she was sent.”
All this is to say that if we interpret the phrase literally and don’t assume a medieval misspelling that there is little evidence for (despite the speculations of the Catholic Encyclopedia and numerous theologians), “missa est” means “she (or it or perhaps even he) was sent (or dismissed).”
My personal best guess is that “Ite, missa est” means “go” (“ite” is the plural imperative – literally, “go, all of you”), “it [whatever ‘it’ or ‘she’ or whatever is] was sent forth [or dismissed].” What that means – “go, it was sent forth” – is up to interpretation.
Whatever it means, “ite missa est” does not mean “go, the mass is ended” or “go, you are dismissed” or any number of other common mistranslations. If “missa” is indeed a medieval corrupt spelling of “missio” in this case, it might mean, “go, it is the sending forth,” but I believe “go, it/she was sent forth/dismissed” is the more accurate translation.
Hope that helps.