Henry Walsh Mahon was born in Ireland in 1809.
He studied at the Royal College of Surgeons, Dublin, the École de Médecine, Paris, and King's College, London. In 1835 he became a Surgeon in the Royal Navy.
Later he was Staff Surgeon to European Pensioners in New Zealand. In retirement he was an honorary member of the Surgical Society of Ireland, and a Member of the British Medical Association. 
Henry W. Mahon was employed as Surgeon Superintendent on two convict ship voyages to Australia:
Henry W. Mahon was initially appointed to the Mexborough convict ship in 1841 . This did not eventuate as John S. Hampton was Surgeon on the Mexborough departing from Dublin on 12 August 1841 and Henry Mahon was employed as Surgeon to the Barossa which departed Sheerness two weeks later.
Barossa to Van Diemen's Land in 1842. The Barossa arrived on the evening of 13th January 1842 with 348 male convicts. (VDL)
Description: Page from the journal of Henry Walsh Mahon showing the effects of scurvy, from his time aboard HM Convict Ship Barrosa
H.M.S. Samarang 1843 - 1846
Henry Walsh Mahon was appointed Surgeon to HMS Samarang 1843 - 1846.HMS Samarang under command of Edward Belcher was used for survey and zoological studies of the East Indian Archipelago, the East Indies, the Philippines and Borneo over a period of four years. The expedition aimed to use every facility at its disposal in the pursuit of science, making observations, collecting specimens and making sketches and drawings of rare forms to help advance zoological knowledge of that part of the world.
Many of the drawings had been sketched by Arthur Adams, a Linnean Society fellow, malacologist, naturalist and editor of HMS Samarang's zoological report. Adams as Assistant Surgeon on HMS Samarang and Henry Walsh Mahon was Surgeon. The drawings of Arthur Adams were included in the Medical Journal of Henry Walsh Mahon. 
Sir Gilbert Blane Medal
Henry Walsh Mahon was awarded the coveted Sir Gilbert Blane Medal in 1846. Sir Gilbert Blane had instituted this prize in 1829 for the best journal kept by a naval surgeon.
In January 1848 Henry W. Mahon was appointed Surgeon in New Zealand. He practised in Howick and purchased land in Onehunga in 1850. He departed New Zealand late 1856, receiving a farewell address from Otahuhu military pensioners.
Medical Register 1865......
Residence: Westport Co. Mayo.
Qualifications Member 1828
Roy. College Surgeons Eng. Member Royal College Phys. London 1841.
Lic. Midwif. K.Q. College Phys. Ireland 1863. 
Plarr's Lives of the Fellows Online MRCS July 25th 1828;
FRCS (by election) Aug 26th 1844;
B ès L Université de France 1829;
MRCP Lond 1841;
LM Coombe Hospital and LKQCPI 1863. 
He died at 88 Bushfield Avenue, Dublin, on March 9th, 1878.
Henry Walsh Mahon
who departed this life
9th March 1878
in the 70th year of his age
'Rest for the weary'...Headstones Mount Jerome Cemetery, Dublin .
Probate of the Will of Henry Walsh Mahon, formerly of Westport Co. Mayo and late of 88 Bushfield Avenue, Dublin M.D. Surgeon Royal Navy who died 9 March 1878 granted 6 April 1878 to Mary Mahon and Meliora Gray both of Bushfield Avenue, widows.
Notes and Links
1) Page from the journal of Henry Walsh Mahon, surgeon aboard HMS Samarang. Watercolour of the Cassia Alata or Acapuleo of Manila, signed Arthur Adams. The journal contains notes on the plant's medical properties. - National Archives UK Flickr
2) Samarang Medical Journal dates from 1 January 1846 to 18 January 1847 contains several drawings of animals and plants; with some interesting remarks on objects of natural history National Archives UK Flickr