de Bourg vs de Búrca

de Bourg

name
  • A topographical surname from Anglo-Norman for someone who lived in a fortified place. 

de Búrca

name
  • A topographical surname from Anglo-Norman for someone who lived in a fortified place. 

How often have the words de Bourg and de Búrca occurred in a corpus of books? (source: Google Ngram Viewer )