Glad that you had a successful trip to Ireland. You said you were helped by "a gentleman in Co Monaghan who knew everything about the genealogy of the county" -- perhaps that was Theo McMahon? He's a great historian and his book "Old Monaghan" is wonderful. Also, Jack Storey is a Monaghan native (now a Belfast resident) who regularly posts historical data on the message boards and helps visitors and researchers. He's worked diligently to transcribe church records in the northern part of the county and offers these on CD.
The RCB Library is the "repository" that you were told about, a 15-minute bus ride south of Dublin city centre. It's the "Representative Church Body" Library of the Church of Ireland -- Go to www.ireland.anglican.org/and click on a search for "library" for more information.
The RCB Library has many interesting books, manuscripts, church registers and records, including early censuses (visitations or returns). You can look up topics in the card file by parish, diocese or county, such as "Clones parish registers," 1682-2001.
The Library has a typed copy of the Groves Manuscripts including petitions, returns, and the 1766 religious census (which has been transcribed in parts, with errors, on various genealogy web sites). "Clogher visitations," for example, include 1622, 1634, and 1673.
The "Returns to the Bishop of Clogher" (Church of Ireland) taken in May 1824 include this type of family data:
Parish: Aghabog Townland: Coraduff Name: Jn. Steenson - 4 daughters, no sons, no servants, 3 people over age 14. Remarks: "In addition to these, there are 30 mixed Families of Seceders and Church Protestants, who have among them 40 Bibles, 39 Prayer Books, and 60 Testaments. I hope shortly to have more Bibles &c distributed through the Parish."
You'll also find some of this type of research material at PRONI in Belfast and the National Archives and National Library in Dublin.
I live in Alabama but have been to Ireland several times and correspond with cousins and other researchers there. Most of my Irish ancestors came from Scotland with the plantation of Ulster, and I've been researching them since the 1970s, taking up my mother's and great-aunt's earlier research of 1920s to 1960s. Will be glad to help you with any other records I've gathered, mostly in Co. Monaghan and Co. Tyrone. Even if not specifically on Fergusons and Wiggins, would be glad to at least suggest possible places to LOOK for more records. Wiggins (Wigham, Wiggams, etc.) by the way, are numerous in southern Tyrone .