Firstly the colour of the mare is red. When Maurya went down to the spring well to give Bartley the piece of cake Cathleen had forgotten to give she saw this vision. It is a belief among the Aran Islanders if a living person comes in contact to any dead relative, a disaster is meant to happen to him/her. This belief can also be seen in the reference to the story of Bride Dara.
The 'red' mare Bartley was riding represented vitality,energy and has a relation to life. On the other hand the 'grey' pony represented lifelessness. Its relation to death is enhanced when Maurya sees dead Michael riding it in new clothes and shoes(a reference to Resurrection). So,to Maurya it was the last proof of a disaster to take place with respect to Bartley's life.