My great grandfather enlisted in December 1915 at the age of 25 years. On his attestation papers there is a question about previous military service which has been answered with "3yrs A.I.R. (4th) resigned". It is definitely an 'R' and not a misshapen 'F'. I've searched the Australian War Memorial site and of course google to no avail. With just the initials there are too many general results and I've tried many variations of what A.I.R could stand for. Could anyone suggest what it could be or other avenues to search?

My guess:
Australian Imperial Regiment
http://www.bwm.org.au/site/Contingents.asp

Yes, I tried that. But from what I can find, the Australian Imperial Regiment fought in the Boer War until 1902 and then seems to disappear from the records. My ancestor would only have been 12 years old in 1902. He would have had to serve his 3 years somewhere between 1908 and 1915, assuming he could only enlist at 18 years old.

My other thought was Reserves, but I can't find any information on whether they existed at that stage. I've also tried searches for Infantry rather than Imperial with no luck.

We didn't really have a regular army at that stage, so all units would have been Militia/Reserve.

Could this have been something to do with the Universal Service Scheme introduced in 1911?

https://www.awm.gov.au/encyclopedia/con ... l_service/

It will be the 4th Australian Infanty Regiment, (1903 to 1908) and then 1st Battalion 4th Australian Infanty Regiment (1908 to 1912)

Orgins to 1st Regiment NSW Rifle Volunteers from 1860.

Succeeded by the 16th Infantry (Newcastle) regiment and theN 2nd Battalion AIF.

Current unit 2nd/17th Battalion The royal New South Wales Regiment.

There is a couple of pictures on the AWM site that gives you some clues and then you just need to search some regiment history to find the rest.