Complex sentence: Seosamh was a less prolific and less fortunate writer than his brother, and was stricken by a severe depressive psychosis in 1935, so that he had to spend the rest of his life--more than fifty years--at a psychiatric hospital.
Split into simpler sentences: Seosamh was a less prolific and less fortunate writer than his brother. He was stricken by a severe depressive psychosis in 1935 and spent the rest of his life--more than fifty years--at a psychiatric hospital.