There are a lot of versions of this song, and several tunes. If you are unsure if this is the tune for the version you are looking for, you'd better ask Big Mick. I notice that Mick supplied lyrics at another thread for the Munster version as recorded by Karan Casey with Solas, Jimmy Crowley and Pádraigín Ní Uallacháin.

There are a few tunes for Níl 'na Lá / Níl sé 'na Lá at the Mudcat Midi page.

Other threads include
Halloween song [sic]
and There's the Day
both have a fair bit of information in them.