Rynd attended medical school at Trinity College Dublin, where he was reputed to be a "wayward" student, known for his busy social life and love of fox hunting.

In 1836 he took a surgical post in the Meath Hospital working alongside William Stokes and Robert James Graves.

In a 12 March 1845 article in the Dublin Medical Press, Rynd outlined how he had injected painkillers into a patient with a hypodermic syringe in on 3 June 1844:[6][7] "Margaret Cox, aetat.

59, of spare habit, was admitted into hospital, May 18, 1844, complaining of acute pain over the entire of left side of face, particularly in the supraorbital region, shooting into the eye, along the branches of the portio dura in the cheek, along the gums of both upper and lower jaw, much increased in this situation by shutting the mouth and pressing her teeth close together, and occasionally darting to the opposite side of the face and to the top and back of her head.On the 3rd of June a solution of fifteen grains of acetate of morphia, dissolved in one drachm of creosote, was introduced to the supra-orbital nerve, and along the course of the temporal,malar, and buccal nerves, by four punctures of an instrument made for the purpose.

After this the pain did not recur, and she was detained in hospital for some weeks, during which time her health improved, her sleep was restored, and she became quite a happy looking person.