Dominic is identified by his Dominican habit and the dog
holding a torch at the center of the composition.
The identification of St. Catherine of Siena is more
problematic. The crown of thorns on her head is normally an
attribute of St. Rose of Lima.
However, there is at least one
other instance of an artist transferring the crown to
another Dominican woman saint.
Furthermore, the way the Virgin is holding her right hand
(and the Christ Child his left) suggests that it once held a
rosary. In that case, the sculpture would refer to the
institution of the rosary, traditionally represented as the
Virgin and Child giving the rosary to Dominic and Catherine (example).
This image in full