Remember that people join the services for many and varied reasons, not just to protect their country. Many times, this was one way of getting fed, as simple as that, though in many times, the services were struggling to provide enough food as well. Some looked on it as an adventure, others as a means of getting away from their home situation, or to give them a 'home'... there are and were, so many reasons.
As well as service records, the museums attached to Military, can also be a great source of information, particularly if you do have some idea of what unit, Battalion, etc. you are tracing, but while they don't hold personnel records as such, they do hold logbooks, personal items, etc.and are usually run by folk who have a passion and a great font of knowledge.
Maybe some of your lost folk may be found by tracing through some of these links....
This link takes you to the National Archives, and gives you many options to search.
British Soldiers pre 1913
British Soldiers post 1913
The Long Long Trail
I like to look for other sites, other than the obvious and direct. This has some great reading.
This a great service as well, free to search, but you do pay to download actual registers and various items. Sometimes the indexes will be all you need.
Commonwealth War Graves
Royal Air Force Museum
Irish Soldiers in the British Army
This is just one of many sites that may help when looking for Irish in the British services.