Dennis Considen's name is included in the Medical Register dated 1780.
He served as Surgeon on the ship Porpoise armed transport in 1781 and 1782 attending to the troops on board during that time.  Among the men killed in an action under command of Commodore Johnstone with a French Squadron commanded by Monsieur de Suffrein on 16 April 1781 in Port Praya Road, St. Jago was Henry Roach who was master of the Porpoise.  Read an account of the battle from the Gentlemen's Magazine.
Appointment to Scarborough
Denis Considen was appointed assistant-surgeon on the First Fleet ship Scarborough in 1788. He arrived on the vessel to take up his duties on 10th April 1787 .
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The Scarborough arrived in New South Wales on 19th January 1788.
He made a study of the natural history of the country. He was prompted by utilitarian as well as by academic motives, for he used the products of indigenous plants to alleviate dysentery, scurvy and other diseases which scourged the settlement. 
From the History of Sydney Hospital by Dr. J. Frederick Watson on the centenary of the laying of the foundation stone of the hospital - In the beginning of February, 1788, the erection of the first hospital was commenced on the west side of Sydney Cove, near what are now known as the Commissariat Stores, George street North; it was completed by twelve convict carpenters and sixteen hired men from the ships. An soon as it was finished it was filled, and the overflow occupied tents around it. Some of the drugs were found to have perished during the prolonged voyage, and others were of inferior quality; but it is interesting to read that the native sarsaparilla proved to be powerfully anti-scorbutic, and an infusion of 'wild myrtle' astringent in dysentery, the honour of these discoveries being claimed by Dennis Considen. 
Dennis Considen made his claim of discovery in a letter to Sir Joseph Banks dated November 1788 -
Mr. D. Considen to Sir Joseph Banks. (Banks Papers.)
Port Jackson, Nov. 18, 1788.
From the intimacy which subsisted between you and my friend, Capt. Charles Hamilton, I have taken the liberty of sending home some birds and a kangaroo skin, properly stuffed, to your care, to be forwarded to him as soon as possible. At the same time I beg your acceptance of five birds and a kangaroo skin - all properly prepared and stuffed. I have likewise sent two living opossums (one for you, the other for Captain Hamilton), and two Animals and beautiful paroquets alive (one for Mrs. Charles Hamilton, the other for your daughter). I sincerely wish they may reach you safe.
Understanding you were a naturalist as well as a botanist, I have sent you some beetles, viz., two species from S. America, and some flower-seeds (such as I could at present collect in this country), and specimens of two sorts of gum, the production of this country, the one red and the other yellow. The first is the red astringent gum well-known in England ; the other, I have taken the liberty of naming the balsam to be of New South Wales. These I have used medicinally, and found them to answer my most sanguine expectations.
I have sent you some of the sweet tea of this country, which I recommend, and is generally used by the marines and convicts. As such it is a good anti-scorbutic, as well as a substitute for that which is more costly. This country produces a variety of flowers and shrubs totally unknown in Europe, and five or six species of wild myrtle, some of which I have sent to you dried. An infusion of the leaves of one sort is a mild and safe astringent in the dysentery. We have a Flowers and large peppermint-tree, which is equal, if not superior, to our English peppermint. I have sent you a specimen of it. If there is any merit in applying these and many other samples to the benefit of the poor wretches here, I certainly claim it, being the first who discovered and recommended them. 
In correspondence from surgeon Thomas Jamison to to Lieut-Governor King regarding his status Jamison referred to his service with Dennis Considen at Norfolk Island - On my arrival in New South Wales, and your being appointed Commandant of Norfolk Island, I was directed by Governor Phillip to accompany you as assistant surgeon to that settlement, where I landed 4th March 1788. Until your return to England in March 1790, I had sole charge of the sick, stores, medicines, etc. On your departure I was superseded by Mr. Considen, assistant surgeon, and had leave to return to Port Jackson; but from the number of people that were then on this island, and many of those that were landed from the Sirius and Supply being sickly, in a consultation held by you, Liut-Governor Ross and Mr. Considen, I was requested to stay to assist that gentleman which I complied with. 
In November 1791, after his time on Norfolk Island working with Thomas Jamison and Darcy Wentworth was finished, Considen was relieved by William Balmain and returned to Sydney.
A son Constantine William and daughter Constantia Cowley were born in New South Wales. Constantia on 1 May 1793. They were the children of Dennis Considen and Ann Cowley, who had been transported for seven years for 'privately stealing' and had come out in the Lady Penrhyn in the First Fleet. 
Denis Considen's name appears on an Index to Deed and Grants for Leases of Land (1792 - 1826) in which he was granted 2 acres near Sydney . This land, granted by Governor Philip was situated between the Church Land and the Ground used as a Brickfield without the town of Sydney. The lease was for a term of 14 years
Return to Ireland
In correspondence dated 14 July 1792, he was granted leave because of ill health . He sailed in the Kitty arriving at Cork in February 1794.
He served in the Army Medical Service as a hospital mate until October when he was commissioned as Deputy-Purveyor for service on the Continent. From 15 July 1796 to 24 June 1799 he was employed as Deputy Purveyor at the Military Hospital, Gosport; and at the Isle of Wight from 25 June 1797 to 24 September 1798. 
In August 1799 he was promoted Purveyor but next March was put on half-pay. This allowance of about £200 a year enabled him both to study medicine at the University of Edinburgh, and to support his two children. On 24 June 1804 Considen graduated doctor of medicine with a thesis entitled De Tetano in which he referred to his discovery of eucalyptus oil. 
The National Archives catalogue notes him being employed as Purveyor to the Forces at Cork and to the Hospital Ship 'Sarah' in records dated 8 Aug. 1805 to24 Feb. 1806
On 6 January 1812 Dennis Considen was admitted a licentiate of the College of Physicians.
He died on 29 September 1815 perhaps at Cumberland St. New Road.
In his Will dated 26 May 1812, he is recorded as Physic of Marylebone St, London. He left all of his estate to Constantia Cowley for her sole use and benefit