When Drake stops by at Bierce's girlfriend, Penny's apartment, she comments on how good his English is, to which Drake replies 'it's perfect - my mother was Irish.' In fact, Patrick McGoohan's mother and father were both Irish.
When questioning Bierce on his sudden reappearance at the embassy after having been missing, Drake at one point mistakenly calls him "Mr. Pearson", though both before and after in the scene he uses the correct name Bierce.
In the earlier episode The Professionals (1964), Drake did in fact investigate the disappearance of a colleague named Pearson, which may possibly account for Patrick McGoohan's slip of the tongue here.