Do sing any song you like the sound of, in French, English, Irish or whatever. You'll never be able to figure out if they are "Celtic" or not. France has a similar linguistic history to Ireland and England(among others). These countries used to have a majority speaking a Celtic language, now they don't. French has taken over decisively in the case of the first one, English in the other two. Such is history. Whether a good thing or not, that's what's happened. All countries have wonderful songs, in all sorts of languages. Enjoy.