 Along with several other "state prisoners" (persons imprisoned indefinitely without charge) Neilson was released in February 1798 following several petitions by influential friends, on grounds of bad health.Upon release he immediately involved himself in the United Irishmen, aligning with the radicals among the leadership who were pressing for immediate rebellion and opposed to the moderates who wished to wait for French assistance before acting.The United Irishmen were, however, severely infiltrated by informers, among them Thomas Reynolds, who kept Dublin Castle abreast of their plans and discussions.