  In quantum magnetism spin dimers are typically associated with spin-singlet
states. To date, $triplet$ $dimerization$ has not been observed in any 3D or
quasi-2D material. With this in mind we here discuss the electronic structure
of the spin-orbit driven $5d^5$ Mott insulator Li$_2$IrO$_3$, a
honeycomb-lattice system with two crystallographically inequivalent Ir-Ir
bonds. From $ab$ $initio$ many-body calculations we find that, while both
Heisenberg and Kitaev couplings are present, the magnetic interactions are
dominated by a strong isotropic ferromagnetic exchange on only one set of
bonds. This causes the formation of triplet spin dimers effectively placed on a
strongly frustrated triangular lattice. The triplet dimers remain protected in
a large region of the phase diagram, suggesting that Li$_2$IrO$_3$ has a
long-range incommensurate magnetic ground state that is pushed by the Kitaev
exchange interactions beyond a standard planar helix configuration.
