A distinction is made between the reference system (the ICRS) and frame (ICRF). The ICRS is the set of prescriptions and conventions together with the modelling required to define, at any time, a triad of axes. The ICRF is a practical realization, and currently consists of a catalogue of equatorial coordinates for 608 extragalactic radio sources observed by VLBI.
The best optical realization of the ICRF currently available is the Hipparcos catalogue. The extragalactic sources were not directly observable by the Hipparcos satellite and so the link from Hipparcos to ICRF was established through a variety of indirect techniques: VLBI and conventional interferometry of radio stars, photographic astrometry and so on. The Hipparcos frame is aligned to the ICRF to within about 0.5 mas and 0.5 mas/year (at epoch 1991.25).
The Hipparcos catalogue includes all of the FK5 stars, which has enabled the orientation and spin of the latter to be studied. At epoch J2000, the misalignment of the FK5 frame with respect to Hipparcos (and hence ICRS) are about 32 mas and 1 mas/year respectively. Consequently, for many practical purposes, including pointing telescopes, the IAU 1976-1982 conventions on reference frames and Earth orientation remain adequate and there is no need to change to Hipparcos coordinates, new precession/nutation models and so on. However, for the most exacting astrometric applications, SLALIB provides some support for Hipparcos coordinates in the form of four new routines: sla_FK52H and sla_H2FK5, which transform FK5 positions and proper motions to the Hipparcos frame and vice versa, and sla_FK5HZ and sla_HFK5Z, where the transformations are for stars whose Hipparcos proper motion is zero.
Further information on the ICRS can be found in the paper by M.Feissel and F.Mignard, Astron.Astrophys. 331, L33-L36 (1988).
SLALIB --- Positional Astronomy Library