Officers from the garda investigation team in the Northern Region this morning began the dig in an area near where the six-year-old disappeared in Cashelard in 1977.
A Garda spokesperson says the search had been planned for some time.
Meanwhile tomorrow, a large crowd is expected to gather in Ballyshannon for the 'March for Mary Boyle'
People donning purple and white ribbons will assemble at the Abbey Arts Centre at 2pm, proceeding through the town and finishing the march at the Garda Station.
The event has been described as a peaceful and non-political demonstration.