Shelton's activities in Ireland brought him to the attention of the English intelligence service. He seems to have been employed in carrying letters to persons in England from Lord Deputy Fitzwilliam at Dublin Castle. However, evidence emerged that he was hostile to the English crown: a letter was intercepted in which he offered his services to Florence MacCarthy, who was seeking to arrange a military intervention by the king of Spain—the Spanish sent an expedition to Kinsale, Ireland in 1601.