The Shackletons are a well known Quaker family (Society of Friends). There are still some Shackletons in Ireland. The most famous one was Sir Ernest the Antarctic explorer. There is more information about them in the book 'A Biographical Dictionary of Irish Quakers' by Richard S. Harrison published by Four Courts Press Dublin 1997 ISBN 1851823042 It does not mention Joseph specifically but would be a useful source as there are not many Shackletons and they would all be connected I surmise.