There were three Johnston's Court and at least two Dignam Court addresses in Dublin city, but I think the two off Gt. Britian St (now Parnell St) are most likely.
Thoms 1848
Dignam's Court, off Gt. Britain St, Civil Parish - St. George
5 Old Houses in Tenements
Johnson's Court, off Gt. Britain St, Civil Parish - St. George
1 to 4 Tenements... entire street (1 to 12) is tenements is 1863
(all the addresses later known as Johnston's are referred to as Johnson's in the edition )
Details or occupants are not given for any of the above in any other Thom's I checked - 1868, '72 or '79 etc
Both the streets appear to have been to the north of the Gt. Britain street, close to each other, and not far from Rutland/Parnell Square.
The Painter's Union Hall that Michael mentioned, survived for about 90 years at the same address, and was close to the jct with York st and nearly opposite St. Peters Church :
27 Aungier St.
1868 - Sarah Humphrey, straw bonnet & millinery warehouse
1872 - Painter's Hall
1879 - Painter's Hall, James Cassidy, secretary
...
1904 - Painters Trade Hall
1927 - Irish National Painters, Decorators, and Allied Trade Unions - Patrick Mulhall General Organiser.
1949 - (similar details)
1958 - 27 to 30 demolished
The address was a car park in 1972
Sibthorpe painting business is listed as early as 1848 and still listed in 1904 ... maybe they were his employer, or he was a contractor for them ?
Shane