@RevPathMLM Photoshop. However doing something like this is possible with other image editing software, like GIMP or paint(dot)NET with the use of plugins for perspective on the flag. This is mostly a lot of manual adjustments.
@archieropes I made this as an edit of another poster, this isn't something existing before.
The original poster is real, dated roughly to 1960 in China. This is an edit of that poster, making it digital, touching it up, changing colours and details, along with adding Connolly in there.
@ManuelitoCat Sorry, I only saw your reply recently.
It's an edit of a lithograph poster. Using image editing software I took the image, reduced it to 3 colours and separated each into layers. Changed the colours and added details as I saw fit, then used an overlay to add the texture.
Original poster by José Bardasano (1910–1979),
"Conferencia Nacional de Juventudes Socialistas Unificadas" (National Conference of Unified Socialist Youth), 1937.