Holiday Inns Inc. said today it has reached an agreement to sell its Dallas-based bus subsidiary Trailways Inc. for about $100 million to a New York investment banking firm, a privately held Pittsburgh company and a former executive of Greyhound Lines Inc. of Phoenix.

The purchasing group includes investment company Gibbons, Green, van Amerongen Ltd.; affiliates of the Hillman Co. of Pittsburgh; and James L. Kerrigan, a former chairman of Greyhound who will become chairman and chief executive officer of Trailways.

Shortly after Holiday Inns made its announcement late today Trailways President J. Kevin Murphy said he had resigned from the bus company because the new owners intend to bring in their own top management. Murphy will remain an employe of Holiday Inns for a period of up to a year until he finds other work.

Frederick G. Currey, chairman and chief executive officer of Trailways, also resigned his position with the bus company but will remain a director of Holiday Inns and president of the hotel firm's transportation group.

Holiday Inns, which plans to reinvest the proceeds of the sale in its hotel, casino and restaurant business, said it would lose about $12 million in 1979 from the deal. The loss reflects the projected loss on the sale of the company, offset by the estimated income of the discontinued operation to the date of sale.