Information from the Ulster Architectural Heritage Society (Reference44) kindly forwarded to me by the Rector of Saint Macartin’s Church of Ireland Enniskillen,( Reverend Courtney), records the architect of sections of this church as Thomas Elliott. It stated that, “ the present building was opened in 1842.” In spite of both the earlier sources referring to him (Thomas Elliott) as the contractor, he was according to Trimble the Enniskillen historian, the one who ‘prepared the plan.’
The description reads that “Except for the addition of a vestry (1862) and an extension of ten feet to the chancel (1889) the outside remains much as Elliott left it, a pleasant, spikey church in the late perpendicular style much favoured by early Victorian architects.”
There are three additional references stated Thomas Elliott as architect.
Reference 46—The Deanery formerly Rectory, 1896
Reference 50— A late Georgian house with shop and entrance
Reference 56— A mid-Georgian house with shop front and attached Ionic columns

The Impartial Reporter January 1924 stated “ Rev Mr Maude, who subsequently became Dean of the Diocese of Clogher, then had to provide an architect and he chose Mr Thomas Elliott of Ballygonnel, father of the late Mr Thomas Elliott, architect and Borough Surveyor of Enniskillen. Mr Elliott’s plans were submitted to the Ecclesiastical Commissioners and approved by them. Mr Maude also chose Mr Elliott to build the church according to his own plan and he executed the work well to the satisfaction of the rector and the Vestry. The work progressed favourably and was ready for the opening ceremony in 1841.”

Alistair Rowan stated in his book,’ The Buildings of Ireland, North West Ulster’ that the design for the Cathedral was either by “the local architect Thomas Elliott of Ballygonnel or by the regional architect to the Ecclesiastical Commissioners, William Farrel with Elliott acting as contractor.”
However, it seems reasonable to assume that one of the Elliott family was certainly the builder and the other was the architect.
The letter of 1814 presents evidence that this Thomas Elliott was either a surveyor or architect or even both as stated above.

Included in these short notes is a letter written in 1814 by Thomas Elliott, with the inserted address of Keelaghan in County Fermanagh. It clearly identifies Thomas Elliott as a surveyor in this instance as he mentions the various surveying or architectural terms such as ‘ calculations, small divisions, casting up every division by triangles and by the squares so they agree.’ This architect was in the employment of Gabriel Montgomery as implied by the letter.
D. Johnston’s name appears in the old letter. He worked with Thomas Elliott for Gabriel Montgomery also.

The Thomas Elliott letter – The Mc Garrigle Collection / Gabriel Montgomery Archive
Ulster Architectural Heritage Society
The Impartial Reporter 1924 (Extract)
The Buildings of Ireland by Alistair Rowan (North West Ulster)
Enniskillen Parish and Town by W.H. Dundas
Thanks to Margaret Kane Enniskillen Library and Reverend B J Courtney (Rector Saint Macartin’s Cathedral Enniskillen).