I'd be interested to see an image of the accented letter. It may be a symbol added as a pronunciation aid, a symbol previously but no longer in use, or potentially the song is in a dialect (although you'd probably see other alphabetical differences in that case). For any of those possibilities, the safest bet is to use the UEB modifier that matches, or use a transcriber-defined modifier if it doesn't match any of the symbols in UEB section 4.2.