Well, if we take Ireland as being 500 km long and the nearest Mars comes to the Earth as 60 million km then the flag would subtend around 500/60E6 = 8 microradians or about 2 arcsec so a Martian Damian Peach would definitely be able to resolve it.
Or our Martin Lewis, for that matter, who quite often produces of images showing surface details on the Galilean satellites of Jupiter, all of which are less than 2 arcsec across.