The Reg district for Clondavodoc would be Milford but oddly there are no marriages registered for that district between 1873 and 1875 (very few in the years before 1873) and very few births and deaths registered in those years either.
It may be worth contacting the GRO in Roscommon to see if they can explain way.
In my experience of other parishes in Donegal, some RC priests didn’t send copies of their marriages to the civil registrar. Not sure whether this was a deliberate policy of obstruction (harping back to the original reluctance of the RC church to participate in civil registration) or whether it was a local phenomenon with priests who just couldn’t be bothered. Either way there do appear to be a lot of early RC marriages in that county that are not in the civil registers.
The next step would be to write to the parish priest (who should hold the only copy of the parish register for 1875) to see if the marriage is recorded there.
