I refer you to my previous post. A huge amount of our fans are part of the Irish diaspora that emigrated to Scotland, including myself. They're free to wave as many Irish flags as they like, and celebrate their connection to Ireland. I've seen Saltires at Celtic games too, but usually it's a green saltire for obvious reasons. Although there were more saltires in the build up to the referendum, if I remember correctly. It's possible to be proud of both your Scottishness, and your Irishness.
Though the proper saltire is seen: