Hi Zapa,
A college's reputation changes over time - I can remember there was real snobbery about DCU degrees in the early 90s, but today I'd say it's a very well respected institution.
I've worked in both TCD and DBS in the past (in neither any more) - the latter wouldn't be anything as prestigious, but I wouldn't assume therefore that the quality of teaching is any less. If anything, DBS have a very mercenary attitute towards their lecturers (many lecturers are let go every year if they get a poor report from students), whereas in TCD, most of the lecturers are full time tenured researchers, for whom lecturing can be an irritating distraction. There are of course some brilliant exceptions, but most TCD lecturers are primarily interested in, and compensated for, their research. This is the same for any research-focussed university.
The DBS part time lecturers almost all work professionally in their field, so the course content does tend to be more applicable to real life and have less "academic" guff than TCD. The DBS lecturers are also compensated based on how good a teacher they are, not on what research they've done this year - it's an important distinction.
So, though it's true that DBS wouldn't currently be as well regarded as the state colleges, that doesn't necessarily reflect on the quality of the teaching, or what you'd get out of a degree, especially if it's one of the evening post-graduate qualifications.
Good luck with your studies in any event!