  The great potential of Dirac electrons for plasmonics and photonics has been
readily recognized after their discovery in graphene, followed by applications
to smart optical devices. Dirac carriers are also found in topological
insulators (TI) --quantum systems having an insulating gap in the bulk and
intrinsic Dirac metallic states at the surface--. Here, we investigate the
plasmonic response of ring structures patterned in Bi$_2$Se$_3$ TI films, which
we investigate through terahertz (THz) spectroscopy. The rings are observed to
exhibit a bonding and an antibonding plasmon modes, which we tune in frequency
by varying their diameter. We develop an analytical theory based on the THz
conductivity of unpatterned films, which accurately describes the strong
plasmon-phonon hybridization and Fano interference experimentally observed as
the bonding plasmon is swiped across the promineng 2\,THz phonon exhibited by
this material. This work opens the road for the investigation of plasmons in
topological insulators and for their application in tunable THz devices.
