1885 Church Wedding record but missing from Civil Registration?

[Wexflyer]

There is a potential reason why it might not be registered.

It was illegal under the penal laws for a Catholic priest to marry a Protestant. To register such a marriage was to provide legal proof of the "crime". I would also note that such a "marriage" was not legally valid - was null and void. That provision was not repealed until sometime in the 1870s/80s.  At least one priest had to flee to the US earlier in the century, to avoid transportation to the Antipodes...

I'd have thought that it was no longer illegal after Disestablishment of the Church of England in Ireland in 1869?

Disestablishment was on 1/1/1871.
It was no longer illegal when the MATRIMONIAL CAUSES AND MARRIAGE LAW (IRELAND) AMENDMENT ACT 1870, S38 came into force.

[Wexflyer]

I don’t think it was anything to do with Penal laws, all of which were long repealed by the date you are interested in.  (The last Penal Law – preventing Catholics from being elected to Parliament – was repealed in 1825). The Roman Catholic church may have had it’s own canonical rules on mixed marriages but that wasn’t anything to do with the Penal Laws.

Hardly. Last provisions of penal law weren't repealed until 20th century. Mixed marriage restriction was repealed by S.38 of the MATRIMONIAL CAUSES AND MARRIAGE LAW (IRELAND) AMENDMENT ACT 1870.

[hallmark]

Plenty of RC registers have Protestant beside a party's name and many of these have been found in Civil Reg by people here looking for kin!

[Wexflyer]

Plenty of RC registers have Protestant beside a party's name and many of these have been found in Civil Reg by people here looking for kin!

Thus proving - if the marriage was performed before the 1870 Marriage Act - that they were in fact not legally married at all!

[Sinann]

Also on Roots Ireland I found a record of David Lees converting and being baptized into the Roman Catholic Church on 26 Nov 1885 (he was born Church of Ireland), not sure if this is relevant though. David followed his father into the army and served 1869-75.

He was baptised RC before the marriage so no mixed marriage difficulties.

[Sinann]

Plenty of RC registers have Protestant beside a party's name and many of these have been found in Civil Reg by people here looking for kin!

Thus proving - if the marriage was performed before the 1870 Marriage Act - that they were in fact not legally married at all!

Don't follow that logic.
Why register an illegal marriage

[aghadowey]

It's not uncommon to find a Catholic baptism shortly before or even on the same day as a Catholic marriage where either bride or groom was not originally Catholic.

[Wexflyer]

Plenty of RC registers have Protestant beside a party's name and many of these have been found in Civil Reg by people here looking for kin!

Thus proving - if the marriage was performed before the 1870 Marriage Act - that they were in fact not legally married at all!

Don't follow that logic.
Why register an illegal marriage

It was less than an illegal marriage. An illegal marriage can be otherwise valid, but such a mixed "marriage" was no marriage at all, in civil law. In Catholic canon law, of course, it was a perfectly valid marriage.

[Sinann]

Plenty of RC registers have Protestant beside a party's name and many of these have been found in Civil Reg by people here looking for kin!

Thus proving - if the marriage was performed before the 1870 Marriage Act - that they were in fact not legally married at all!

Don't follow that logic.
Why register an illegal marriage

It was less than an illegal marriage. It was no marriage at all, in civil law. In Catholic canon law, of course, it was a perfectly valid marriage.

I still don't understand.
If a marriage is registered surely it's legal.
I could understand if it was in the church register only but when it has a Civil cert isn't it legal.