Irish fashions at the time for well to do families were identical to those worn in England. The photograph was taken in 1905. There is good reason to believe that both the younger children are probably boys based on their rough housing with the wheelbarrow. Notice the two girls further back in the painting. One of these girls has a hat and the other is in a white dress, sitting on the ground and talking.
Little girls are very verbal and typically cluster together to chat.
This can be seen even today in school playgrounds. Boys in conntrast seem to prefer activities involving motion, which is also observalbe in playgrounds today. Of course it impossible to tell with certainty. The clothes do not provide any real clues as to gender at the age of these children. It does seemmlikely that the two children in dresses, including the one in a long back-buttoning pinafore, are boys.