I found the photo in my grandmother’s collection. She was born in 1920 in Dublin – Catholic family. She only had one brother and I don’t recognise him in this photo – and we do have photos of him as a boy.
That photo looks like the Boys Brigade to me. This was/is a Christian Youth Organisation in the UK and Ireland. See http://www.boys-brigade.org.uk for similar pictures They were not Catholic though.
I wonder if the outfit was slightly different in Ireland? You could send their archivist (they are bound to have one somewhere) a copy of the picture and ask them what they think….If this photo depicts one of their Irish units they will be quite interested to see it. You could also ask where in Ireland/ Dublin the unites were based and if they have any records, such as membership records which may have survived. Worth a go?