UK Military Indexes, 1920–1971 collection.
The indexes include soldiers born between 1866 and 1901 and there are, inevitably, many Irish men among the 371,000+ individuals named in the collection.
Being an index, the information offered is relatively light but it's sufficient to identify the service man. Each entry should include the soldier's initial, surname and date of birth, together with their service number and rank, and a Ministry of Defence Reference. The latter allows the researcher to apply to the MOD for further details of the soldier's military service. A link is provided to guide researchers making such an application.
I'm glad to see this index online. Some years ago I applied to the MOD for the service record of an ancestor who had fought in the South Africa (Boer) War and served in India, and was still in the British Army in 1919, when his medal card showed he received an award. He must have left the service shortly after the award because the MOD could find no later discharge and could not, therefore, furnish his service records. Had this index been available, I wouldn't have wasted my time applying (there was a non-refundable charge of £30 at the time, if I remember correctly, AND I had to provide his 1949 death certificate).