That really posed two questions. Who is Mary A? And who is James Louis' mother? Here's what has transpired.
I had written a letter to the Washington County Historical Society, asking if they could look for any old news articles about John. Lo and behold, a few days ago, I received his newspaper obituary from 1894. Here's a transcription:
"A veteran of this place, died at the residence of his son, Mr. James McLaughlin, on Saunders street, at 2 o'clock on Sunday morning. The deceased had not been in his usual health for about two years past. He became worse last August and has since steadily declined. Funeral services were held at 10 o'clock Tuesday a.m. at the church of Our Lady of Angels, and was very largely attended, the members of Post Tanner being present in a body. The Rev. Father McLoghlin officiated. The interment was in Smith's cemetery. The deceased was born in Mansfieldstown, Parish of Darver, County Louth, Ireland, in 1826, making him 68 years of age at the time of his death. He married Miss Bridget Hoy about 44 years ago, who died five years subsequent to their marriage. Mr. McLaughlin came to this place when a comparatively young man and has since resided here. He is survived by one sister, Margaret, and one son, Mr. James McLaughlin, who is married and has a family of bright and promising children. Mr. McLaughlin was a veteran of the late war, he having enlisted in Company F, 169th Regiment, in 1862. He was a color sergeant during the engagement at Fort Fisher and was the first to plant the flag on the fort. In the engagement at Cold Harbor he received a severe wound by being struck in the right shoulder blade with a piece of a shell. The wound undoubtedly produced permanent injury to the lungs. At Fort Fisher he saved the life of Gen. Alden, rescuing him from a perilous position. Gen. Alden was at his side when he received the terrible wound at Cold Harbor and was able to give valuable evidence in assisting him to obtain a pension. Gus. Vaughn was Captain of the company in which Mr. McLaughlin served and both men were together when Capt. Vaughn was shot and killed at the explosion of the Petersburgh mine. Mr. McLaughlin dragged his body from the wreckage and debris and assisted in the burial. The following well known residents of this place were members of the company: Daniel Cummings, James M. Waters, Caleb Earl, Julio Benjamin and Eben Blin. The deceased fought in thirty-two battles, remaining in the Service from the time of his enlistment until the close of the war in 1865. He was one of the charter members of A.H. Tanner Post G.A.R. whose membership at the time of its organization numbered thirty. Although the post has at present a larger membership than when first organized, it is not so strong in numbers as a few years ago. These latter years are making havoc in its ranks. Time, while it does not cause such immediate mortality is more deadly than volley or broadside. This soldier, whose courage and tact enabled him to pass through the ordeal of so many battles finally was compelled to surrender his life through an attack of hemorrhage of the lungs. If any man could justly claim the title of American citizen it was this man. He was honored and respected by his comrades and his remains were borne to their last resting place by the following members of the Post: P. Casey, John Davey, Edward Marshall, James O’Reilly, C.B. Tinsley, and James T. Boyle."
Notice that one specific citation, "He married Miss Bridget Hoy about 44 years ago, who died five years subsequent to their marriage. Mr. McLaughlin came to this place when a comparatively young man and has since resided here. He is survived by one sister, Margaret, and one son, Mr. James McLaughlin, who is married and has a family of bright and promising children." John McLaughlin had been married and widowed early in life, having had one son. This explained all the census years during which he lived with a son and his sister Margaret. I still haven't figured out who the mysterious Mary A. is, but I have no doubt I shall figure it out. By the way, one of those "bright and promising children" was my grandfather, William Clinton McLaughlin, known to me as "Grampa Will."
For now, I am pleased to have found the marriage record of John McLaughlin and Bridgett Hoy (Hoye), dated 18 August 1849. They were married in Whitehall, New York. This means that John migrated to the US and settled in Whitehall before 1849. I suspect he traveled with his sister. Of course, I'm thrilled to know precisely where in Ireland this branch of my tree took root. It's the tiniest county in Ireland, on the East coast, directly below the Northern Irish border. There's so much more research to do...