Proquest Disssertations & Theses (Global) also known as Index to Theses, is the most comprehensive source of doctoral theses from the UK, Ireland and other countries around the world with coverage from 1716 onwards. Some of the references are full-text; others provide a preview of the abstract and table of contents. You can find this on the A - Z of Databases.
Theses in Progress in Commonwealth Studies is an annual listing of MPhil and PhD research being carried out at UK universities.
OpenGrey is System for Information on Grey Literature in Europe, an open access system to 700.000 bibliographical references of grey literature (paper) produced in Europe including technical or research reports, doctoral dissertations, some conference papers, some official publications, and other types of grey literature.
RIAN, for Irish research, is a portal of open access research publications from seven, Irish university libraries. Searches can be filtered to masters' and doctoral theses.