I have Thomas Donlan's Civil War service record and pension files.
Thomas was born in Ireland in 1820; d. 9 May 1890, Watertown Mass.
His father and mother were Patrick and Winifred Donlan.Patrick was my 3xgreatgrandfather.
Thomas was married on 1 Jan 1857 at St Columbkil's church on Market St in Brighton, Mass just across the Charles River from Watertown where he he was to live with wife Catherine Grady.The best man at his wedding was Martin Donlan.
I have found a Martin Donlan living in Watertown Mass b. 1829 in the 1860 cenus.Subsequently, the family moved to Cambridge.The 1870 census lists his wife and children, but does not list him.
Were Thomas and Martin brothers?Intuitively it seems so, but unless I can find evidence that their father was Patrick, this is only speculation.