Is it possible to write and defend a dissertation on work for a private company? I am in an oddball situation where I have amassed a number credits beyond a masters at University in the US. I am currently working in a research setting, but I am aware that without the PhD by career ceiling is somewhat low. I do not want to leave and go back full time because I like my job and I want to stay in industry anyway. 

Hypothetically, assuming no IP issues, is it possible to write a thesis on a private work related topic? I would be returning to the same school that gave me the masters degree and I would self fund, but I am curious to know if I would face a lot of friction if I asked. The research topic is not a core strength for the department but I just need to affiliation etc... I work with all PhDs from respected institution, who would be pseudo advisers. Getting a PhD on my day to day work would be advantageous since I already have a sound background and I know my degree would be marketable.
There are certainly ways to get a PhD while working on a project supported by a private company.

It will almost certainly require enrolling in a PhD program, doing the required coursework (if any), passing the necessary exams, assembling a committee, proposing the dissertation, then writing and submitting it. There are few credible institutions that will look at a body of work done outside the institution, go "Well, that's it then." and award you a doctorate.

You would be well served in talking to potential advisors and chairs within the department about their feelings on the matter, as well as your current supervisors - they may not view a PhD earned while working for them as the same as an external PhD ("If we hire you, we get new expertise") so it might not actually resolve your career ceiling problem to your satisfaction.

But in principle, there's nothing prohibited (barring a university's guidelines) in working on research driven by private industry. Though additional complexities may arise - what if the company doesn't want you to publish one of your dissertation papers, for example? Get everything settled, preferably in writing, beforehand. And recognize that working full time and getting a PhD is a tough road.