Beneath a naked bluff on a mile-long Boston Harbor island known chiefly for its antiquated jail lie the unmarked graves of about 4,100 early immigrants whose American dreams ended in death. Most were famine-weakened Irish who fled their starving country in the mid-1800s only to be deemed ``undesirable'' and detained on Deer Island, where they died and until now were largely forgotten. ``They symbolize all immigrants that have come to Boston and to America,'' Mayor Raymond Flynn said this week. ``They may have been poor, probably didn't have any family and weren't among the elite. ``But still every respect should be afforded to their memory.'' Residents of Boston, home to more Irish-Americans per capita than any other U.S. city, are resurrecting the city's missing chapter. Deer Island's low-security jail is scheduled to close in 1991 and local business, labor and other groups plan to have the gravesite landscaped by 1995, the 150th anniversary year of the potato famine. The mayor has announced plans to erect a Celtic cross on the island and a statue near Fanueil Hall as part of the city's ``Great Hunger Memorial Project,'' the nation's first memorial to Irish famine victims _ and victims of hunger around the world. The graves on the island about four miles from Boston's wharves came to Flynn's attention several months ago, after construction workers turned up bones at a new sewage treatment plant site. ``You think of them getting this far, coming across the Atlantic and being able to look over and see the city ... only to end up in an anonymous pauper's grave,'' said Francis J. Costello, the mayor's adviser on Irish-American Affairs. ``We have to put a proper finish on this.'' Many of the thousands of immigrants held on Deer Island eventually made it to a new life on the mainland. But thousands more were buried in shallow, lime-lined trenches that held eight or 10 bodies each. Their anonymous graves tell the story of immigrants who saw but never reached America's shores. Ireland's population of 8 million was cut in half by fatal disease, starvation and emigration after a devastating plant disease hit the country's staple potato crop in 1845. During the next 10 years, more than 152,000 Irish fled to Boston, many of them tenant farmers forced to move on when they no longer could afford to pay rent. The Yankees were unprepared for the influx of penniless, weak and diseased immigrants, many of whom were near death after a month or more aboard the crowded ``coffin ships'' they had taken across the Atlantic. ``As a precautionary measure to ward off a pestilence that would have been ruinous to the public health and business of the city,'' a quarantine hospital was established on Deer Island, according to a Massachusetts Senate document dated 1848. Paupers and inmates also were held on the island because officials deemed them, like the Irish and a fraction of immigrants from other countries, ``undesirable within the core urban area,'' according to city documents. ``It was really a human dump heap,'' said Dennis P. Ryan, author of ``Beyond The Ballot: A Social History of the Boston Irish.'' Several rough-hewn wooden crosses stood for years on the bluff near the graves, but by about 1970 they had disappeared or disintegrated. A lone white Celtic cross also was erected amid the yellow rapeseed, but that too is gone. ``It wasn't a deep dark secret,'' said Deborah Cox, president of the Public Archaeology Laboratory Inc. in Pawtucket, R.I. ``I think it was just one of those things people don't pay attention to and then eventually forget,'' said Cox, whose company surveyed Deer Island before construction began on the new sewage treatment plant in late 1988. Boston's archivist, Edward Quill, has begun researching death records and handwritten weekly logs kept by physicians at Deer Island's quarantine hospital. For years the records have gathered dust, now they'll be available to genealogists and other researchers. Several Flynns have turned up among the 872 names Quill has traced. ``It could well have been any immigrant group that has come to our country,'' said Mayor Flynn. ``That's why we need a symbol, a decent resting place ... so their history won't be forgotten.''