Help with info on Great Grandfather in British Army

[Jag]

Hi I'm trying to find out about my Great Grandfather who was Irish but was a career soldier in the British Army until he retired aged 55 in the early 1900's.
His name was Dennis O'Neill & he served a long time in India.
I know it's going to be difficult with so little info but could anyone tell me where to begin searching?
Thank you

[Wholley]

Do you know what regiment?Any chance you have his Army number?
In the early 1900's Britain was busy getting out of India,so if you can find a phot in uniform post it here might wake some of the older among us from the afternoon snooze

[timex]

You can apply for his Military records, not sure how you do it though. Try a search on here and you may get some pointers.

Good luck