Cronin was born in 1801 in Cork, Ireland before moving to Dublin for health reasons in about 1826. In Dublin he studied medicine at the Meath Hospital, and later utilised his medical ability on Anthony Norris Groves' pioneering mission to Baghdad - after the death of his wife in 1829, Cronin went with Groves to administer medical support including dealing with an outbreak of plague. While in Persia and later India, he also dealt with cholera and typhus using homeopathic principles.
Cronin returned to England in 1836, where, as a medical practitioner, he became an early adopter of homeopathy in the UK - Cronin is estimated to be the fifth such practitioner to introduce homeopathy. He was a member of the English Homeopathic Association, and in 1858 he became the last man to become a Lambeth MD before the Medical Act 1858 abolished this particular qualification. Cronin remarried and settled in Brixton where he lived until his death in 1882. Cronin's eldest son Eugene also took up homeopathic practice, and another of his sons became honorary dentist to the London Homeopathic Hospital.
Originally a Roman Catholic, when Cronin moved to Dublin he sought membership with various dissenting churches in the area but was only admitted as a visitor. He began meeting with other Christians including Anthony Norris Groves, John Gifford Bellet and John Nelson Darby, whose conviction that the ordination of clergy was unnecessary and unscriptural, as well as his dispensationalist and premillennialist theology which later became principle tenets of the Plymouth Brethren movement.
He remained faithful to this movement all his life, but one of his last actions was to precipitate a split in the already fractured movement. When a number of members of a failing assembly at Ryde had stopped attending the meeting, he travelled down and met with some of them and celebrated the Lord's Supper. A furious row erupted with different assemblies disagreeing about which side was right and therefore to be supported, with Darby, who had privately sympathised with him, attacking him in the strongest terms. The row escalated but was not resolved.
- Mosley, C., ed., Burke's Peerage, Baronetage & Knightage, 107th edition Wilmington: Burke's Peerage (Genealogical Books) Ltd, 2003), volume 1, page 876
- H.H., The Origins of the Brethren, 1825-1850, Pickering & Inglis, 1967, p. 37
- Bradford, T.L., Pioneers of Homeopathy, 1898
- Epps, J., Homoeopathy and Its Principles Explained: And Its Principles Explained, English Homoeopathic Assn., 1850, p. 314
- Epps, J., Homoeopathy and Its Principles Explained: And Its Principles Explained, English Homoeopathic Assn., 1850, p. 231
- Mason, A.S., "Wasn't It Exciting!" A Compilation of the Work of A. Stuart Mason, p. 203, Royal College of Physicians, 2004, ISBN 1-86016-206-1
- Monthly Homeopathic Review, Vol. 26, p. 193.
- Rowdon, H.H., The Origins of the Brethren, 1825-1850, Pickering & Inglis, 1967, p.37
- Neatby, William Blair (1901). A History of the Plymouth Brethren. p. 138. Archived from the original (PDF) on 10 March 2001. Retrieved 2012-06-11.
- Biography by Dr Thomas Lindsey Bradford, from Pioneers of Homeopathy, 1898
- Entry from Burke's Peerage