Have you considered that they may have left Ireland, either moved to Britain or emigrated to U.S, Canada, Australia etc.? Another career move for young Irish men was joining army. A large proportion of soldiers in British Army in 19thC were Irish. A career move for a young Irish woman was moving to Britain to be a domestic servant or to work in a factory. There were also itinerant Irish hawkers and labourers in Britain, some of them seasonal workers. Many lived in lodgings, sometimes in common lodging-houses with a lot of other Irish people. There were encampments of navvies, some with families, near big construction projects. Names of many were incorrectly recorded at census time.
As to your theory that they may have died as a result of the Famine, local history resources may be the way to go. The local Poor Law Union was responsible for destitute people. Poor Law Boards of Guardians made reports. Members of clergy, charities and other concerned people wrote to newspapers and to Parliament about what they'd witnessed. Where were they born?
You may be interested in a book called "Annals of the Famine". It's a contemporary account by an American woman who was travelling around Ireland in1840s. Disposal of the dead is one of the topics she reflects on. There are many other books about the Famine period. This week's episode of Feargal Keane's RTE series "History of Ireland", currently being shown on BBC tv, covered the 19th century.