Complex sentence: The gang had intended to seize Austin Darragh, owner of the Institute of Clinical Pharmacology, but Darragh had moved from the house three years previously and O'Grady was his son in law.
Split into simpler sentences: The gang had intended to seize Austin Darragh, owner of the Institute of Clinical Pharmacology. However Darragh had moved from the house three years previously, and O'Grady was his son in law.