Searching the early Roman Catholic parish registers for your Ottawa Valley ancestors can be a bit confusing, at least until you get a sense of the lay of the land. Basically, if you want to find all relevant baptismal and marriage records for your family, you’re almost certainly going to have to search more than one register, and probably at least a few.
Before the formation of regular, local parishes, many Catholics in the Ottawa Valley were served by travelling missionary priests, who made periodic visits to a given village or township to perform baptisms, marriages, and less frequently, burials, and who then recorded the performance of these sacraments in any number of possible parish registers, sometimes many miles away from an ancestor’s address.
So, for example, the parish of St. Isidore at South March (now Kanata, in Carleton Co., Ontario) was originally known as the “Mission of March;” and the “Church Registry of Births, Marriages and Deaths in the Township of March” (later the parish register for St. Isidore) begins in mid-1861, with the first recorded baptism (9 June 1861) that of John Fahy/Fahey, son of William Fahy and Margaret King. However, there were certainly Catholics in March township well before 1861, and indeed from its early settlement in the 1820s. So where are their sacramental records to be found, for the pre-1861 period? Well, many of the RC baptisms and marriages which took place at March township in the 1830s, 1840s and 1850s were recorded in the parish register for Notre Dame Basilica (which was not yet a basilica) in Ottawa (which was not yet Ottawa, but rather Bytown [until 1855]). And some of the records for Catholics of March township, circa late 1850s and early 1860s, are also found in the register for St. Patrick’s Basilica in Ottawa (and the earliest of these records predate by a couple of decades the construction of the beautiful church on Kent Street that was elevated to a basilica in 1995.) (The first marriage recorded in the register for St. Patrick’s Basilica, Ottawa, btw, is that of my 2x great-grandparents John Lahey and Margaret Jane Killeen, both of March township). For early Catholic records for ancestors residing in March township, you might also want to check the registers for St. Philip’s, Richmond; St. Michael’s, Corkery; and Fitzroy Harbour Mission.
To return to the question of Catholic parish records for residents of Arnprior (McNab township, Renfrew Co., Ontario):
First, some of those (Ireland to Canada emigrants, say, but also Québec/Lower Canada to Ontario/Upper Canada migrants) who later settled in Arnprior (or in Renfrew County more broadly) can first be found (or at least, their parents or grandparents can first be found) in the Bytown/Ottawa area. So for the first generation or two of your Catholic migrants-to-Arnprior ancestors (be they Irish or French Canadian), don’t overlook the Ottawa area parish registers (e.g., the register for Notre Dame Basilica, Ottawa [Bytown]).
Second, and this point applies to the Ottawa Valley area more broadly, political borders (between Upper Canada/Ontario and Lower Canada/Québec, for instance) did not necessarily neatly coincide with RC ecclesiastical boundaries. Or perhaps I should put that the other way around. Basically, the Church had its own thing going on when it came to the care and feeding of its flock, and the Ottawa River (which roughly divides Ontario from Québec) was not always and necessarily a definitive marker of relevant jurisdictions. The diocese of Pembroke, for example, which certainly now (and ever since about 1882, I guess) includes the parish of St. John Chrysostom at Arnprior, emerged from the Vicariate of Pontiac, which included the various missions and parishes of Pontiac Co., Québec, along with numerous missions and parishes in Renfrew Co., Ontario and in the northern regions of Counties Frontenac, Addington, and Hastings, Ontario, as well. Some of the records for your English-speaking (but probably Irishly-origined) Catholic ancestors in Renfrew Co. may well be found in Québec parish registers, in other words.
The parish register for St. John Chrysostom, Arnprior begins 8 December 1867, and was generally kept in very good order by meticulous French and French-Canadian record-keepers. One of my favourite record-keepers ever, by the way, is Rev. Father Alphonse Chaine, parish priest at Arnprior for about 40 years, who was born in a tiny village in the French Alps in 1836, and search me how he ended up at the bog end of nowhere, in a lumber town that had just barely been surveyed, in Renfrew Co., Ontario. Father Chaine had a charm which escaped the confines of the strict formulae for RC parish records. I love how he used the occupational designation “shantyman” for some of my Irish ancestors, as if this were a perfectly respectable calling, and I love even more how he rendered this term en français as “voyageur.”
And before that (before 1867, I mean), the Mission of Portage du Fort (later St. Jacques le Majeur/St. James the Greater) in Pontiac Co., Québec (begins April 1851) was also known as the Misson of Arnprior [Renfrew Co., Ontario], and I have certainly found records pertaining to my Irish Catholic emigrants-to-Arnprior ancestors in that French-Canadian parish register.
And I’ve also found records for Arnprior-based ancestors in the following registers:
Fitzroy Harbour Mission (Carleton Co.)
St. Michael’s, Corkery (Huntley township, Carleton Co.)
St. Patrick’s, Mount St. Patrick (Renfrew Co.)
St. Jacques le Mineur/St. James the Lesser, Eganville (Renfrew Co.)
St. Peter Celestine, Pakenham (Co. Lanark)