FindMyPast has added a number of military collections sourced from The National Archives (UK). You can see the full list and details here.
The two listed below are the most likely to be of interest to those with Irish-born ancestors who served in the British Army. You'll need either a UK or World subscription to view the records*.
British Armed Forces, First World War Soldiers' Medical Records
The collection holds more than 212,000 names and includes details of when, where and how individual servicemen were wounded, and where they were treated. The search page lists the medical facilities and hospitals represented in this record set.
British Armed Forces, First World War Widows' Pension Forms
Transcripts provide the following details about the serviceman and his family: service number, regiment, cause and date of death, spouse's name, marriage year and children's names, while images may provide further information about attestation and service. This collection of just under 9,000 records represents less than 2% of the original set; the rest were destroyed.
*This strikes me as unfair when the records cover an era when Ireland was still part of Britain. The records are every bit as Irish as they are Scottish or Welsh or English.