Hello Pippa
I've done some research into Captain Samuel Phillips ...
As you say, he was Ensign (by purchase) into the 18th Foot from cadetship at the Royal Military College in Aug 1868. He was Lieutenant in Oct 1871, Adjutant of the regiment from Aug 1873 - May 1874, Captain Jun 1880 and Major in Aug 1888. He seems to have retired not long after that.
In the Afghan War he served with his regiment on the Khyber line in 1880. They suffered a lot from heat and cholera, and this might explain his medical leave to England c.1881. He rejoined the 1st Battalion in India in Feb 1882.
In Jan 1884 he was appointed as Adjutant to the 4th North Tipperary Militia. Later in life, c.1899, he was High Sheriff of Gaile, Tipperary.
Gaile, County Tipperary, is where he was born in 1849 - and according to a public family tree on Ancestry he also died there - in 1919. He married Helen Clementina Fraser (née Gilchrist) in 1885 and they had three children: Richard, Moira and Aileen.
Samuel Phillips can be found on the 1871 census (Raglan Barracks, Devonport), the 1891 census (Leinster Gardens, Paddington) and the 1911 census (Oakley Hall, Basingstoke).
I hope this is of interest.
Best wishes -
Garen