Complex sentence: In 1647 he accompanied the Marquis of Ormonde during his Irish campaign, and remained with him until the retreat of the royal army from Dublin to Drogheda, when he made his way to the Isle of Man, and thence crossed to Wales.
Split into simpler sentences: In 1647 he accompanied the Marquis of Ormonde during his Irish campaign, and remained with him until the retreat of the royal army from Dublin to Drogheda. He made his way to the Isle of Man, and thence crossed to Wales.