Ó Néill spent over thirty years on the continent as a military officer in the service of the Spanish. During that time, he learned the art of war and earned acclaim as an excellent general, notably for his defense of Arras in 1640. He actively communicated with fellow expatriates and watched the affairs of England and Ireland with a desire of returning and participating in a rebellion. Ó Néill formed a brotherhood with his compatriots in the Irish regiment and they encouraged him to return to Ireland and lead a rebellion.