He left Poitiers in 1754 and entered a Jesuit school at St Omer to train to be a Jesuit but decided the life of a priest wasn't for him. He returned to Galway the following year. He converted to Protestantism in 1764 and two years later was called to the Irish Bar.

Kirwan went to London in 1777 where he studied, amongst other subjects, chemistry, physics, meteorology and geology. He returned to Dublin in 1787. His wife Anne had died after eight years of marriage, leaving him with two daughters. He bought the house at Cavendish Row where he remained until his death.

In addition to his main interests of chemistry and geology, Kirwan devoted a great deal of time to the study of the weather. He built an experimental meteorological station at Cavendish Row and many Irish farmers refused to plant crops before consulting him.

Kirwan had many strange and unusual habits. He hated flies and he paid his manservant, Pope, a bounty for each fly that he killed. He also had an irrational fear of catching cold and insisted that Pope keep a fire blazing all year round. Before leaving the house Kirwan would warm himself at the fire before donning a heavy coat and several scarves in order to conserve the heat. He would then emerge from the house at a brisk trot and refuse to stop for anyone even for a minute.

Kirwan suffered from dysphagia, a disease that prevented him from swallowing his food properly, so he always ate alone and his diet, for the most part, consisted of ham and milk. However, he didn't live in total isolation and he entertained guests at his house every Wednesday and Friday evening.

Guests were expected to arrive at 6pm and at 7pm Pope would remove the brass knocker from the front door and no latecomers were admitted. Kirwan would entertain his guests from the comfort of his fireside chair, still wrapped in his cloak and with a wide-brimmed hat on his head.

Visitors were expected to leave at 9pm sharp when Kirwan would rise from his chair and look pointedly at his pocket watch. If anyone failed to get the hint, Kirwan would remove his shoes and breech-straps and this usually had the desired effect.

Kirwan died in 1812 at the age of 79 and was buried at St George's Churchyard in Hill Street. Ironically, he died as a result of starving himself while trying to get rid of a cold.