THERE appears to be a slight hitch with an exhibition of Irish art (don't laugh, it's true) that is being held at the Grill Room of The Excelsior. Some of the works that will be shown appear to belong to the Realism School of Art - ie, they display uncooked meat. Excelsior general manager Liam Lambert - an Irishman to boot - has now decreed that only ''suitably garnished works'' will be exhibited, so as not to offend the sensibilities of his customers. We are informed that as a result some of the ''untasteful'' works of art have been placed in cold storage, which at least will ensure they do not go off. The exhibition will run until the end of April. But, at a special showing of the complete meaty works on Friday, you may judge for yourself whether one man's meat is another man's, er, artwork.