Flags.
Both the St George, the Three Lions, AND the irish tricolour ( with a Palace logo of course ) , would be good. Lets show Shelbourne that we got more irish fans than them. Therefore we are a bigger club in Ireland than them.
the Union Flag...union jack.... has always been a little bit controversial on these shores. But for the life of me i cannot say why. It doesnt really bother me. But suffice it to say you would show discretion about wearing Palace colours in a Brighton Housing estate. The Union Flag can elicit a similar reaction among republican types, Sinn Fein voters, people with Palestine flags, Glasgow Celtic shirts, etc. Its probably fine to fly the Jack inside the stadium, but just remember that a small drunken fella wandering a dodgy neighbourhood alone, in the evening, with a Union jack t-shirt....probably not a good idea.....you get the picture. I expect the Shelbourne fans will be fine and well behaved - more interested in football than politics.
And for the sake of balance, be reminded that although there are very many Micks who support Palace, there are probably also a decent number of Nigels who support Shelbourne. Such is the interconnected nature of the modern World.