“Rmo” is an ecclesiastical abbreviation for “Reverendissimo” (Most Reverend), and “D” is the abbreviation for “Dominus/Domino” (Lord). The term “Reverendissimus Dominus” would be translated into English as “The Very Reverend Monsignor”. And “Ordinario” refers to the ordinary, which is almost always the local bishop.

Given that the ordinary’s name is not provided in the sentence, and “The Very Reverend Monsignor” is used only in front of a name, you might want to modify this somewhat, either to something simple along the lines of “The ordinary”, or else adding in the full name (if you have it) to end up with “The ordinary, the Very Reverend Monsignor ___ ___, …”

“Rmo D. Ordinario dispensante”, an “ablative absolute” construction, could be translated in a variety of ways depending on the rest of the sentence. One generic way to translate the beginning of this sentence would be “With the ordinary, the Very Reverend Monsignor ____ ____ granting a dispensation from the forbidden time, …”

As for “Rmo D Can.”, I stands for "Reverendissimo Domino Canonico", and presumably refers to a person with the rank of cathedral canon. I imagine that this would be the official issuing the marriage certificate (or perhaps a permission to be married) on behalf of the ordinary.

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Note added at 9 hrs (2021-10-16 18:19:52 GMT)
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Of course, "With the ordinary, the Very Reverend Monsignor ____ ____" could be abbreviated as "With the ordinary, the Very Rev. Msgr. ____ ____".

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Note added at 10 hrs (2021-10-16 19:21:31 GMT)
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The ablative absolute requires a noun plus a participle or adjective. Here “dispensante” is the participle, so “ordinario” must be a noun, not an adjective. The Ordinarius dispensat, hence “ordinario dispensante”. (And the fact that "Ordinario" is capitalized reinforces the fact that it is a title of a position.) Other examples would be “Christo dispensante”, “Domino dispensante”, “ecclesia dispensante”, “concilio dispensante”, and “iudice dispensante”.