I would recommend an examination of any surviving service/discharge/pension records for the Royal Engineers to see if they provide a more precise address in County Tyrone, such as parish or town, for John Kelly when he signed up. Many such military records can now be searched online on websites such as Ancestry and FindMyPast.
I see also that the Royal Engineers Museum has produced a 2-page leaflet providing information on how to research ancestors who served for the Royal Engineers. It can be viewed at
https://www.re-museum.co.uk/wp-content/uploads/2015/07/How-To-Research-Your-Family-History.pdfKelly is the second commonest family name in Ireland, after Murphy, and part of the reason is that, at least seven and possibly as many as ten distinct septs of the same name arose in different parts of the country. Kelly is the sixth most common name in Ulster, and the third most numerous in Counties Derry and Tyrone.
The Kelly sept of Derry/Tyrone trace their lineage to Eogan, son of the 5th century High King of Ireland, Niall of the Nine Hostages, who ruled from the Hill of Tara, County Meath. By tradition, Eogan and his brother Conall Gulban conquered northwest Ireland,c.425 AD, capturing the great hill fort of Grianan of Ailech in County Donegal.