Working as a physician to the prominent O'Donnell clan during the Nine Years' War, he may have followed their chief to Spain after the Siege of Kinsale, where he spent two decades practicing medicine.

[2] After the Irish confederacy's defeat at the Battle of Kinsale, clan chief Hugh Roe O'Donnell travelled to Spain to secure reinforcements.

[22] Historians Kate Newmann and David Murphy have suggested that Ó Glacáin left Ireland after the defeat at Kinsale, due to his support for O'Donnell.

[25][27] Around 1628, he worked as a travelling plague doctor, treating victims at local hospitals in towns such as Fons, Figeac, Capdenac, Cajarc, Rouergue and Floyeac.

[2][29] MacCuinneagáin states that Ó Glacáin "gained high esteem and general consideration because of the devotion which he showed in braving the contagion to succor the sick.

Ó Glacáin describes conducting four postmortems where he noted the occurrence of petechial haemorrhages which "covered the surface of the victims' lungs and also the swelling of the spleen".

To this end, the university's senate asked Giorgio Scharpes (Professor of Medicine, 1634–1637) to write a report on Ó Glacáin, whose fame by then spanned all of Europe.

He is famous because during the plague in these regions of France during the years 1627 to 1629 he was very helpful and in the year 1629 he produced a book whose title is Tractatus de Peste ... and I invite you to read this book to understand exactly why Mr. Glacáin is valuable and why he is still teaching in the University of Tolosa ... About his teaching he is well estimated because he is a good philosopher, good in fighting against his enemies that accused him of being a magician; his book can confirm that he was not a magician ... Mr. Glacáin knows Greek very well ... talking about the other questions ... from a letter from Mr. Glacáin where he says he would really like to serve the University of Bologna, I can understand that there will not be any problem for the salary and for him to come.

[12] Two Irish residents of Bologna, Gregory Fallon and Jesuit Phillip Roche, wrote commendatory verses prefixing the second volume.

In collaboration with them, he wrote eulogistic poems in Latin to Pope Innocent X, titled Regni Hiberniae ad Sanctissimi Innocenti Pont.

[5] The final volume of Cursus Medicus begins with his eulogy, as written by Peter von Adrian Brocke, Professor of Eloquence at Lucca: With healing art he arms us to repel, dire troops of agues and of fevers fell, whatever ills the patient may endure, known or unknown, unerring is his cure..."[2][33]