Brennan Ireland 1901 Religion

Of the 14,673 people in Ireland in 1901 with the surname Brennan religious identification was predominantly Roman Catholic.  
(Figures supplied from the search engine on the free Census website (www.census.nationalarchives.ie/) ...and they don't add up!)

Roman Catholic 14,041  
Church of Ireland 254
Presbyterian 160
Methodist 27
Church of England 11
Baptist 4 (Dublin)
Plymouth Brethern 3 (Kerry and Dublin)
Congregationalist  2 (Antrim)

There are a small number of people described as Presbyterian living in Antrim. (approx. 21)

Approx. 100 others seem to be Roman Catholic with such a variety of spellings that the search engine doesn't recognise  the words.