Several years ago for a client, I was able to trace his ancestors who were Plymouth Brethren in Newport, County Mayo. So starting with no research guides and no clue; I dug deep. What a fascinating piece of religious history.
If you are exploring the 1901 or 1911 Census of Ireland online: www.census.nationarchives.ie you will find these families as; Christian, Christian Brethren, Brethren, Plymouth Brethren, undenominational Christian Protestant, Exclusive Brethren, or Open Brethren. In 1901, there were around 5,000 Brethren in Ireland.
I want to also refer you to an in-depth article I wrote “The Irish Plymouth Brethren” in The Septs, 32, #2 (April 2011), pp. 89-92. The Septs is the journal of the Irish Genealogical Society International in South St. Paul, Minnesota which is online: http://irishgenealogical.org/sites/default/files/memsepts/2011322.pdf
In short, in 1827, a questioning Church of Ireland minister serving in County Wicklow, named John Nelson Darby (1800-82) joined other Protestants questioning the very idea of denominationalism. Out of their Bible studies arose new insights, which would influence conservative evangelical Protestantism.
However, most are unaware ideas such as Dispensationalism and The Rapture were first developed by John Nelson Darby.
Brethren records as a whole, have not been deposited. One excellent collection is the “Christian Brethren Archive” housed at the John Rylands University Library in Manchester, England; www.library.manchester.ac.uk/searchresources/guidetospecialcollections/brethrenThis collection has many directories which can help you pin point where congregations met.
With these few resources, I promise that you are far more prepared than I ever was when I started my quest.