Researchers looking for information about their immigrant ancestors' arrival in Boston have access to a free online resource that is currently a work in progress: Boston Passenger manifests from 1848-1891. This is the period during which many of the Irish immigrated to the United States, including many of my ancestors.
The huge collection of these passenger lists are currently being placed online thanks to the Massachusetts Archives. According to their website, "Massachusetts officials started recording the names of immigrants who arrived by ship in 1848, a procedure that continued until 1891, when federal record-keeping programs superseded those of the state." Passenger lists for ships arriving into the port of Boston are being added to this online database in stages. The completed database will eventually include the over one-million immigrants who arrived through this port during these four decades.
I haven't yet found an ancestor there, but I plan to keep searching the database as more lists are added.
Thanks to Jerry Reeds' Free Genealogy blog for highlighting this online resource.