Complex sentence: On 25 January 1915 (his eighteenth birthday) O'Sullivan enlisted into the Royal Munster Fusiliers, and later transferred to the London Regiment and served with the rank of lance corporal during the First World War and lost a leg at Ypres in 1917.
Split into simpler sentences: On 25 January 1915 (his eighteenth birthday) O'Sullivan enlisted into the Royal Munster Fusiliers, and later transferred to the London Regiment. O'Sullivan served as a British Army lance corporal with the London Regiment during the First World War and lost a leg at Ypres in 1917.