In 2010, his granddaughter Nora researched his military service record and generously shared this information with Clare Museum. The records show that Cornelius enlisted in the British Army on 4th September, 1899. At the time of enlistment the following information about him was recorded:
Age: 19 years and 7 months
Height: 5 feet 5 ¼ inches
Weight: 175 lbs
Chest measurement: 38 inches minimum
Distinguishing Marks: two scars on his right elbow
Following his enlistment, Cornelius was assigned the rank of Sapper and given the Regimental number 3636. He was then sent to the 12th Field Company, Corp of Royal Engineers to start his training as a soldier. On 12th July 1900, having completed his training and having served a total of 311 days in Britain, he was deployed with his unit to Wei Hai Wei in China. The Boxer Rebellion was underway and it is likely he saw action during his time there, which lasted one year and 263 days. He was transferred from China to Hong Kong on 16th May, 1902 and was on duty there for 45 days before being posted to Gibraltar. He would spend the next two years and 331 days at this Mediterranean outpost carrying out duties in support of the Anglo-Boer War in South Africa.
Cornelius arrived back on British soil on 12th April, 1905 and on 26th May the following year he was promoted to the rank of Sapper 1st Class. He re-entered civilian life on the 4th September 1911 and was transferred to the 1st Class Army Reserve. As a reservist he received a gratuity of £8-72d.
At the outbreak of the First World War Cornelius was mobilized, along with thousands of other reservists. He entered Kilworth Camp in County Cork on 6th August, 1914 and was deployed to France with 12th Signals Company, Royal Engineers, on 9th September. During the war it is understood that his duties included surveying the location of enemy mines.
Medal Index Cards indicate that he was awarded the 1914 Star and clasp for service ashore in France and Flanders between 5th August and 22nd November, 1914. The clasp indicates that he was under fire between those dates. Accompanying this were two small silver heraldic roses that were issued to those recipients who were entitled to a clasp. This was dispatched to him in Ennis on 6th May, 1920 along with his British War Medal, and an Allied Victory Medal.
Cornelius Shanahan was discharged from the Army on 22nd March, 1918. He returned to Ennis and made his living as a painter, a family tradition that could be traced back four generations to 1790. His descendants are still involved in the painting and decorating business to this day.