The Department of Posts and Telegraphs was often referred to as “P agus T” – the “agus” being Gaelic for “and”. This government department was the procurent body for the Irish Army many years ago. It is therefore not surprising to see this logo on the reverse of some earlier Irish military buttons.
I have seen this exact type of button on Irish Volunteer Corps uniforms in both the National Museum and also in private collections. It is also believed that the Irish Army Construction Corps wore this type of coloured button in the 1940’s.
All buttons are approx 25mms in diameter, however two are slightly smaller by about 2mms.