Complex sentence: Prior to the outbreak of the Civil War, Dowling was establishing himself as an able and successful entrepreneur and had been involved with a predominantly Irish militia company which served a more social than military role in Houston society.
Split into simpler sentences: Prior to the outbreak of the Civil War, Dowling had made a name for himself himself as an able and successful entrepreneur. Among other things, he and had been involved with a predominantly Irish militia company which served a more social than military role in Houston society.