He was a council member for the Royal Institute of the Architects of Ireland from 1842 to 1849, and vice president from 1849 until his death.
Farrell “was one of several architects to be attacked in the satirical poem, ‘An Essay on the Rise and Progress of Architectural Taste in Dublin…by Nicolson Numskull, Esq.’ which was published in 1832: ‘With rosy gills - round as a little barrel, A first fruits architect - see Billy F-rr-ll, Who thrives and fattens on anothers brains, He toils and labours - Billy counts the gains.’”[2] His sons joined his practice around 1833 or 1834 and his practice's name was changed in 1839 to "William Farrell & Son."
Arthur Thomas Farrell, another son, attended Trinity College, Dublin in 1830.
In the 1830s, it is thought that James Joseph McCarthy (1817–1882), architect of among other works the Roman Catholic St. Macartan’s Cathedral, Monaghan (1861–1892), apprenticed at Farrell’s office.
Other apprentices included Sinclair Bates in 1809, John Louch in 1813, Parke Neville, and Sandham Symes.