Complex sentence: Colborne became high commissioner of the Ionian Islands in 1843, and having been promoted to full general on 20 June 1854, he became Commander-in-Chief, Ireland in 1855.
Split into simpler sentences: Colborne became high commissioner of the Ionian Islands in 1843. Promoted to full general on 20 June 1854, he became Commander-in-Chief, Ireland in 1855.