WO 16/2917 is a reference to a muster book for the Guards. So not likely to be correct.
The 6th Rifle Brigade was a Militia regiment in Longford or Longford and Westmeath depending upon date. Findmypast have attestation papers for the Militia. They were part-timers, serving for six years. They did 49 or 84 days of training on enlistment followed by 1 or 2 months training a year. One condition was that, in the event of a general mobilization, they would be made full-time and used for home defence to allow a Regular battalion to serve overseas. In 1899 they changed the conditions to allow some Militia battalions to serve overseas during the Boer War. Either in South Africa itself or guarding Boer prisoners.
One problem is that Militiamen tended not to record the fact on census or BMDs. So if his attestation paper does not survive the only other resource are the muster books in Kew. They are not online and you will need to come up with a more accurate date. Before spending lots of money researching military resources you should obtain as many BMD certificates as you can find from the Irish GRO. They only charge four Euros for a certificate on plain paper, which is all you need. One of them may give a regiment.
30-40 Regular battalions would have served in Athlone/Mulligar over say a 20 year period. So this route is not feasible.
Ken