Dillon was a captain in Wentworth's 'new army', which was recruited in Ireland to help put down a rebellion against King Charles I in Scotland. This force was disbanded after an outcry against arming Irish Catholics in the Parliaments of England and Scotland. When it disbanded in 1641 Dillon was authorised, with his own £1,000 ("2009: £"), to raise a regiment of the demobbed soldiers for the Spanish Army. However, this plan was interrupted by the outbreak of a Catholic rebellion in Ireland in October 1641. The brewing conflict lost him his investment and later that year he joined the revolt of his fellow Catholic gentry.