We theoretically study the coherent dynamics of Dirac fermions at the surface of a 3D topological insulator (Bi2Se3) in the field of an ultrafast laser pulse. In the presence of the ultrafast pulse, which has a femtosecond time scale that is less than the characteristic electron scattering time in these materials, the electron dynamics is coherent. Because of the gapless dispersion relation, the electrons dynamics is highly irreversible, which is dramatically different from dielectrics (fused silica, quartz, and sapphire). Due to irreversibility, finite conduction band population does exist after the pulse ends. Within two-band k.p Hamiltonian, which includes cubic momentum hexagonal warping terms, the residual conduction band (RCB) population in the reciprocal space is highly anisotropic. The distribution of RCB population in the reciprocal space strongly depends on the polarization of the ultrafast laser pulse.