Mixed C of I Catholic wedding late 1800s?

[Wexflyer]

I would also have considered it in light of a Scottish custom since these were Presbyterian marriages in Ulster with a long history of travel between Ireland and Scotland.

I am simply saying that if this was a common Catholic and Presbyterian practice, then isn't it obviously an Irish practice?

[Jon_ni]

it certainly seems to have been a common R.C practice too, likely accentuated by the few R.C buildings about coming out of the Penal Era and that what is termed irregular marriages in Scotland were also present. Perhaps all non-Episcopalians. I guess the emphasis has been put on 'Scottish' due to the fact the practice continued there throughout the 1900's whist it ceased in England and Ireland with civil registration.

https://www.cotyroneireland.com/misc/marriage.html

We have been persuing dissenters marriage regulations with curiousity the more familiar statement being:
" where land and property interests were involved marriages performed under the aegis of the Church of Ireland...  ensured legitimacy and legality. Parish registers were kept and used to denote ties of kinship, and this was important not only in marriage but also in courts of law..."

[mackers]

Well my first query fairly started an interesting debate. As it happened I imagined that it might be the case that the COI and the Catholic church were Episcopalian in nature ergo the marriage was able to go ahead. It was also the case that the same situation was the reason I cannot find a marriage for my Great great grandfather and his known Catholic wife.

Seems these marriages were pretty regular in those days. As I discovered that 3 COI brothers married 2 Catholic sisters, plus the Catholic sisters brother married their 2 husbands COI sister all in Saint Anne's. Which suggests the Catholic Church didn't seem to carry out such marriages around the late to early 1800 to 1900s? Or did they?

[Jon_ni]

Which suggests the Catholic Church didn't seem to carry out such marriages around the late to early 1800 to 1900s? Or did they?

You will sometimes see an adult baptism followed by the R.C marriage or annotation on the marriage record to that effect or about dispensations. Per the very start a Register Office followed by a C of I marriage (especially in St Anne's or similar larger cities) would be a much more common choice by the bride & groom.

Episcopalian relates to the Anglican church worldwide, not to the R.C. Church.
https://www.anglicancommunion.org/structures/member-churches.aspx

[Wexflyer]

Seems these marriages were pretty regular in those days. As I discovered that 3 COI brothers married 2 Catholic sisters, plus the Catholic sisters brother married their 2 husbands COI sister all in Saint Anne's. Which suggests the Catholic Church didn't seem to carry out such marriages around the late to early 1800 to 1900s? Or did they?

They certainly did. Might not have advertised it though, pre-1870, as it was technically illegal, with the priest subject to transportation.

As an aside, or tangential to your original question, but just to give you a flavor of what went on:
The last Countess of Anglesey (of the Annesley creation), married as her second husband a Talbot of Castletalbot Co. Wexford, and had a son by that marriage.
In her will, she (or, rather, her lawyer), went to great lengths to avoid calling her second husband her husband, or her son, her son. The only possible reason is that she married in a Catholic ceremony, so that technically her marriage was null and void, and her son was a bastard. The son inherited the bulk of her estate.

Interestingly, her son by her first marriage, the future Earl Mountnorris, was also a bastard, at least in England. The British house of Lords so determined when it refused to acknowledge him as Earl of Anglesey, on the grounds of bigamy by his father (the last Earl of Anglesey, of the Annesley creation). So her first marriage was also held to be null and void - but only in Britain.