What is Thomas Moore's "The Harp that Once ThroughTara's Halls" about?

This poem is about a couple of things.

First of all, it is about how things that are famous and important at one point come to be dead and irrelevant later on.  It is the idea that even the famous and important die and become nothing.

Second, this is a poem about Irish nationalism.  This poem was written at a time when England ruled Ireland.  The poem uses the ancient seat of Irish kings -- Tara -- as a symbol for Irish independence.  It is saying that Irish independence will survive as long as "some heart indignant breaks."