Later in the same year he took part in the Siege of Cork where he first served with Marlborough, then an Earl. It appears that it was during this action that Cadogan, although only a junior officer, attracted the attention of his future commander by his conduct. Following the climatic victory at the Siege of Limerick in 1691 he continued to serve in Ireland for three years having decided to become a professional soldier rather than return to his law studies. In 1694, he purchased a Captaincy in Erle's Regiment, which was then based in Flanders as part of the Nine Years' War with France.