Sweeny was commissioned brigadier-general of volunteers, Nov. 29, 1862. In the Atlanta campaign, he commanded the Second Division of the Sixteenth Corps, Army of the Tennessee. At Snake Creek Gap he took possession of the “Gap” with his division twenty-four hours before the arrival of supporting cavalry, and held it in spite of the desperate efforts of the enemy to dislodge him. Subsequently he took part in the battle of Resaca, and forced a passage across the Oostenaula river, at Lay’s Ferry, where he fought a successful battle which resulted in Gen. “Joe” Johnston’s retreat southward. He also took part in the battles of Dallas and Kennesaw Mountain, the actions at Nickajack Creek, Ruff’s Mills, Rome Cross Roads, Calhoun’s Ferry and other engagements.
At the battle before Atlanta, July 22, 1864, his division drove the enemy back with great slaughter, capturing four battle flags and nine hundred prisoners. This was the day that General McPherson was killed—he had shared Sweeny’s tent the previous night. At the conclusion of the battle Gen. Frank P. Blair, commanding the Seventeenth Army Corps, who had witnessed the repulse by Sweeny’s division, rode down at the head of his staff to where Sweeny stood and grasping him warmly by the hand, said: “Sweeny, I congratulate you. You have saved the army of the Tennessee!”
General Sweeny was one of the Guard of Honor in charge of the remains of President Lincoln when they lay in state in the city hall, New York. On Aug. 24, 1865, Sweeny was mustered out of the volunteer service, and was in command of the post at Nashville, Tenn., to October of that year.
An ardent lover of the land of his birth, General Sweeny longed for the day and the hour when, on the field of battle, he might meet the common foe of his adopted and of his native land. Even while lying disabled by his wounds in Mexico, he had made arrangements to cast himself into the then expected struggle for Irish independence, but the opportunity was not afforded him. In 1856, while he was stationed at Fort Pierre, Nebraska territory, he wrote to his family: “I see they are making another movement for Irish independence. I hope it will amount to something this time. E——, how would you like me to embark in such an undertaking? We might accomplish great things—do deeds that our children could point at on the page of history with pride—perhaps help to pull a sinewy tyrant from his throne, and raise a prostrate people from chains to liberty.”
It was not, however, until after the close of the Civil War that the opportunity arrived. In 1865 the Fenian congress assembled in Philadelphia to decide upon the best means to adopt to obtain the liberation of Ireland. General Sweeny appeared before this convention and submitted to it his plan for the invasion of Canada. He also offered his services to aid in carrying out the plan, both plan and services being accepted. He was appointed secretary of war of the Fenian Brotherhood, and commander-in-chief of the Irish forces with the rank of major-general; William R. Roberts, a prominent merchant of New York and afterwards United States minister to Chili, was appointed president.
In 1866, General Sweeny was offered, by the Mexican government, the grade of general of division in the Mexican army, which he declined. He was finally retired from active service May 11, 1870, with the rank of brigadier-general, U. S. A. He was wounded so often in battle that on one occasion, after an engagement, General Grant met him and laughingly said: “How is it, Sweeny, that you have not been hit? There must be some mistake. This fight will hardly count unless you can show another wound!”
In a sketch of General Sweeny published some years ago, Junius Henri Browne, war correspondent of the New York Tribune, says: “Sweeny is, as his name indicates, an Irishman by birth, but an American to the backbone. No soldier was more devoted to the Union cause, or fought more valiantly for the integrity of the republic.”
General Sweeny was twice married. His first wife was Eleanor Swain Clark of Brooklyn, N. Y., daughter of John and Hepsabeth (Paddock) Clark. In both the paternal and maternal line, she was descended from the Coffins, Barnards, Folgers, Husseys, Macys and Swains, who were among the ten purchasers of Nantucket in 1659. She was sixth in descent from John Swain, 2d, son of John Swain, 1st, and Mary (Weir) Swain, the first male white child born on the island (in 1664), and seventh in descent from Benjamin Franklin.
General Sweeny’s second wife—who survives him—was Eugenia Octavia Reagan of Augusta, Ga., daughter of Dr. Francis Washington Reagan and Sarah Cecelia (Refo) Reagan. Her paternal ancestors were among the first settlers of the present states of Virginia, North Carolina, Georgia and Tennessee. Through her paternal great-grandmother, Mary Dandridge of Virginia, Mrs. Sweeny is descended from Dorothea Spotswood Dandridge, the second wife of Patrick Henry, and Martha Dandridge Custis, the wife of George Washington.
After retirement from the army General Sweeny virtually withdrew to private life. The last public act of his “long, varied and useful career,” was to use his personal aid and influence to secure a fund for the erection of a home at Austin, Texas, for disabled Confederate veterans.