If s miles is the distance to Dublin, then s=12v where v mph is the outbound average speed.
s=(v-85)t where t is the time for the return journey.
So 12v=(v-85)t, 12v=vt-85t, v=85t/(t-12).
There is insufficient information (for example, the return time) to quantify the outbound speed, v.
EXAMPLE
If t=13hrs, v=1105mph. If t=14hrs, v=595mph. If t=13.5hrs, v=765mph.