With the Society of Friends in general, William Forster was deeply concerned about the famine in Ireland caused by the failure of the potato crop in 1846. He visited Ireland with his son (William Edward) in 1846 and 1847 to investigate the conditions there prior to the formation of a Quaker committee for relief.

William's health was poor in later years, but he still made an additional visit to America with other Friends (including his brother, Josiah) to present an anti-slavery address to the President and to Governors of the States. He met with President Pierce in October 1853, but was not encouraged by the response. He continued to meet with other people with influence.

In January 1854, William became severely ill while staying in Tennessee. After a few weeks, he died, aged 70. He is buried in the Friends' burial ground at Friendsville.