What is the meaning of song harp that once through halls?

The phrase "harp that once through Tara's halls" comes from Thomas Moore's Irish melody "The Last Rose of Summer". The song is about a lone rose that has survived the rigors of winter, just as Ireland has survived many hardships. The harp is a symbol of Ireland, and the line "harp that once through Tara's halls" recalls the ancient days when Tara was the seat of Irish royalty. The song is a lament for the passing of time and the loss of Ireland's former glory, but it also expresses a sense of hope for the future.