Even as a small boy, Michael Hirsch loved visiting cemeteries.

His family would take him to visit his great-grandmother’s grave, and soon he came to see cemeteries as museums containing the stories and secrets of the past.

Over the years, Mr. Hirsch, a historian and genealogist, came to feel a sort of kinship with the dead and a duty to care for their graves.

Now 68, he has visited more cemeteries than he can count, cleaning headstones and often honoring people according to their religion, leaving stones on Jewish gravestones and palm crosses for Christian ones during Easter.

One day last November, Mr. Hirsch arrived at Most Holy Trinity Cemetery — 23 acres in the Bushwick area of Brooklyn, tucked alongside the elevated subway line that carries the L train to Canarsie.

Some 25,000 people are buried at Most Holy Trinity, many of them working-class immigrants from Germany who were parishioners of the nearby Catholic churches.

Their lives were modest and so were their graves, marked not with expensive granite but with wooden crosses or metal markers in the shape of traditional gravestones.