Ella Young, in her Celtic Wonder Tales, included a story, “How the Son of Gobhaun Saor Shortened the Road”, that mentions the nine sacred woods (although not by that name).

a brief excerpt…

Balor’s Son set out and the Son of Balor’s Sister with him. Balor’s Druids sent a wind behind them that swept them into the country of the De Danaans like a blast of winter. They came to the house of the Gobhaun Saor, and the wife of the Son came out to them. “O Woman of the House,” said they, “we have a message from the Gobhaun Saor.” He is to light a fire for Balor, and he sent us to ask you for boughs of the oak, boughs of the ash, boughs of the pine tree, boughs of the quicken, boughs of the blackthorn, boughs of the hazel, boughs of the yew, boughs of the whitethorn and a branch of bog myrtle. “You are to give us,” he said, “a white stone from the door step of a Brugh-fer, and a black stone from the door step of a poet that has the nine golden songs.”