  We present results from JCMT sub-mm observations of sources selected from the
{\sl ISO} FIRBACK (Far-IR BACKground) survey, along with UKIRT near-IR imaging
of a sub-sample. This gives valuable insight into the brightest $\sim$10% of
galaxies which contribute to the Cosmic Infrared Background (CIB). We estimate
the photometric redshifts and luminosities of these sources by fitting their
Spectral Energy Distributions (SEDs). The data appear to show a bimodal galaxy
distribution, with normal star-forming galaxies at $z\simeq0$, and a much more
luminous population at $z\sim0.4$--0.9. These are similar to the ultraluminous
infrared galaxies which are found to evolve rapidly with redshift in other
surveys. The detectability threshold of FIRBACK biases the sample away from
much higher redshift ($z\stackrel{>}{_{\sim}}1.5$) objects. Nevertheless, the
handful of $z\sim0.5$ sources which we identify are likely to be the low-$z$
counterparts of the typically higher-$z$ sources found in blank field sub-mm
observations. This sub-sample, being much more nearby than the average SCUBA
galaxies, has the virtue of being relatively easy to study in the optical.
Hence their detailed investigation could help elucidate the nature of the
sub-mm bright galaxies.
