An bhfuil aithne agat ar Bhrian -- Do you know Brian
An bhfuil .... agat means Do you have ...
aithne is used for knowing people;
a fhios is used for knowing something in general,
eolas for particular knowledge, of a subject for example.
See An bhfuil a fhios agat above for examples.

ar Bhrian -- literally "on Brian"

ar -- on is a simple preposition or réamhfhocal simplí
and aspirates the following consonant if possible so Brian becomes ar Bhrian