The largest Protestant denomination in Canada is the United Church of Canada (UCC). It was created in 1925 through a union of the Methodist Church of Canada, over half of the Presbyterian Church in Canada congregations, the Congregational Union of Ontario and Quebec, and the Association of Local Union Churches. The Canada Conference of the Evangelical United Brethren Church joined the UCC in 1968. The UCC has emerged in the tradition of Liberal Protestantism.
The logic behind the uniting of these denominations was practical. Small towns across Canada may have had a Presbyterian, Methodist and Congregational church. Trying to find ministers in rural Canada was a problem. For communities on the prairies, they had already begun uniting prior to 1925 in an Association of Local Union Churches.
This affects your Irish Protestant research. Records for the old denominations are now part of a UCC congregation. They may still be held at the local congregation, deposited at one of the regional conference archives, or at the main UCC Archives in Toronto: www.united-church.ca/local/archivesThe “Archives Directory” on the website provides links to the conference archives. Some UCC congregation records have been microfilmed and are at the Family History Library. However, most are still in Canada.
When you are conducting your Irish immigrant research, and you note the religion in one of the Canadian censuses, the next step is to perhaps think in terms of the UCC to locate and have the appropriate records searched. The Presbyterian registers are trickier as many congregations did not join the UCC. Those records will not be at a UCC archive.
From my own experience, it’s worth your time to track down UCC records. It may be within the local record that the birth place in Ireland is preserved for an immigrant ancestor.