He was associated for a time with the Sheares brothers and Lord Edward Fitzgerald, the United Irishmen but, being dissatisfied with their programme, he cut himself adrift and began to study medicine.

In 1796 he came to London to attend the Hunterian or Great Windmill Street school of medicine, and he became an occasional pupil at St. Thomas's and Guy's hospitals.

During almost the whole of his residence in Dublin Macartney was subjected to a very singular exhibition of petty persecution and open insult at the hands of some members of the board of Trinity College.

An ill-used and greatly misunderstood man, "he was," says Professor Alexander Macalister, an expert anatomist and a philosophical biologist far in advance of his period.

His description of the vascular system of birds has in many respects not been surpassed, and his account of the anatomy of mammals may be read with more profit than many modern works.

He gave, too, the first satisfactory account of rumination in the herbivora, and he discovered numerous glandular appendages in the digestive organs of mammals, especially of rodents.