While collecting the records on my recent September 2011 visit to Dublin, Enniskillen and Belfast, I noted the following in the Census of Ireland and other sources:
http://www.census.nationalarchives.ie/
1901 273 McBriens in Co. Fermanagh
1911 234 McBriens in Co. Fermanagh
2011 86 McBriens with Telephone Numbers listed in Co. Fermanagh*
By contrast, the Dundas family had:
1901 165 Dundas in Co. Fermanagh
1911 167 Dundas in Co. Fermanagh
1962 55 Dundas Voters in Co. Fermanagh (Rev. W.H. Dundas, "Enniskillen-Parish and Towns")
2011 18 Dundas with Telephone Numbers listed in Northern Ireland*
2011 12 Dundas with Telephone Numbers listed in Co. Fermanagh*
*Note: Telephone listings do not reflect those choosing to be unlisted or those with mobile phones.