Depending on the source, his name is spelled either William Mullens or William Mullins.
From, Historic Structures Report, Appendices: Clubhouse, Brown Cottage, Moorhead Cottage, and Clubhouse Annex, Prepared for the National Park Service:
"William Mullins [sic] was born in Ireland, near Dublin, in 1824. He studied civil engineering at Trinity College in Ireland and obtained an excellent reputation in Ireland as a draftsman. He achieved the position of Chief Civil Engineer of Public Works.
In 1848, Mullins [sic] came to America and became employed as civil engineer of the Genessee Valley Canal where he remained for a while. Then he moved to Steubenville, Ohio where he became involved in railroad contracting on the old Cleveland and Pittsburgh Railroad. There he met and married Lucy Bustard in 1857.
During the Civil War, Mullins [sic] became associated with the Pennsylvania Railroad and was of great help to the government in transporting troops and supplies. Around 1863, he was promoted to purchasing agent of the Pennsylvania Railroad Company and lived in Allegheny City in Pittsburgh. He remained in that position until his death.
Mr. and Mrs. Mullins [sic] had three children; Annie Esther born March 1858, William James born August 21, 1860, Edwin Stanton born March 13, 1869. William Mullins [sic] was a patron of the fine arts and a scholarly man who collected a large library. He died August 19, 1893 in Cresson, where he was taken to take fresh mountain air in hopes of recovery, due to a tumor in his stomach."