why does sir lancelot sing "tirra lirra"?
"Tirra Lirra" is basically a sort of traditional song in the world of English Poetry, where it is the song of the lark. A lark is a bird that spends mos of its time flying up high. In this sense the bird is "free".
Therefore I think Tennyson is trying to portray that Sir Lancelot is indeed free, where the Lady of Shalott is not.