A possible marriage was in the Indiana, Marriage Index, 1800-1941 at Ancestry.com for Neil O'Donnell and Mary O'Donnell in Hancock County 1869. A paper copy of this marriage was received from the Hancock County Clerk's office with a turn-around time of one week.
Without other identifying information, I can't be sure that this is the same couple from my tree. No town, no ages, no parents, no witnesses.
The first child, Rose, was born in 1870. The 1870 federal census in Center (Greenfield), Hancock County, Indiana, enumerates a couple that is probably my Neil and Mary O'Donnell, plus Neil's brother, Charles.
In the 1880s, the family relocated to Howard County, where Rose O'Donnell married Frank Lungren in 1889 and Agnes O'Donnell married Cornelius Carey in 1902. These marriages are online (free). Again, no towns, parents, ages, or witnesses.
The Carey-O'Donnell marriage is for this branch. Father Charles M Carey, son of Agnes O'Donnell, edited the book of poems by his uncle, Father Charles L O'Donnell.
Additional records that may yield clues include Catholic church records and death records.
If anyone is familiar with research in Indiana, please let us know if there could be recordings of marriages at the town or state level that yield distinguishing information.