Sorry - not enough coffee so far today!
I studied with someone who was doing a part-time Masters and a part-time job, but didn't come across anyone attempting it with a full-time job. It should theoretically be more manageable, but you'll still have some timetabled activity even if you go for a research-based Masters, so it may depend on how flexible your employer can be with hours/shifts. Some of them will also be wary of an employee taking up a course which will potentially a) reduce their focus on their paid work and b) ultimately result in them leaving. Have you been able to gently test the waters yet? With a supportive employer you have a fighting chance, but with an employer who doesn't see it as being in their interests to co-operate, you could find life very awkward. When I applied to university for my undergrad degree, I didn't dare tell my employer until I handed in my notice - I would've been passed over for all sorts of opportunities. I was told point blank that I wouldn't have been given my annual bonus (for successful projects I'd already completed) if they'd known I was going to resign. If I'd done anything part-time, there's no way I could have told them about it.
From a practical point of view you shouldn't have a problem with something like library access. My Masters uni library was open until midnight Monday to Saturday and also had good opening hours on a Sunday (althought slightly shorter). They were open 24 hours a day in the run up to undergrad exam season, although it was standing room only at that point. You should be able to find library opening hours for your target uni, on their website.
Depending on your field, you might also find that much of your theoretical material is available online. All of the major journals are now available online and you'll get access to these via your uni. This can make studying from home feasible at some points, as well as reading papers in lunch hours at work.