Conversation so far:
What was Robert Stewart, Viscount Castlereagh's early life like?
Robert Stewart, Viscount Castlereagh had recurring health problems throughout his childhood.
What else resulted from Robert Stewart, Viscount Castlereagh's childhood illness?
Robert Stewart, Viscount Castlereagh was sent to The Royal School, Armagh, rather than to England for his secondary education indirectly due to childhood health problems.
How was Robert Stewart, Viscount Castlereagh's time in school?
Robert Stewart, Viscount Castlereagh attended St. John's College, Cambridge, where he applied himself with greater diligence than expected from an aristocrat.
Did Robert Stewart, Viscount Castlereagh have other accomplishments at school besides great diligence?
Robert Stewart, Viscount Castlereagh obtained first class in his last examinations at St. John's College, Cambridge.

Question: What did he do after returning to Ireland?
Self-contained rewrite: What did Robert Stewart, Viscount Castlereagh do after returning to Ireland?