It's actually called "If I Was A Blackbird" in O'Lochlainn, not to be confused with "The Blackbird" which appears in the second volume (More Irish Street Ballads). The words are:

I am a young maiden and my story is sad
For once I was courted by a brave sailor lad
He courted me strongly by night and by day
But now my dear sailor is gone far away

CHORUS:
If I was a blackbird I'd whistle and sing
And follow the ship that my true love sails in
And on the top riggings I'd build there my nest
And pillow my head on his lily white breast

He promised to take me to Donnybrook fair
To buy me red ribbons to bind up my hair
And when he'd return from the ocean so wide
He'd take me and make me his own loving bride

His parents they slight me and will not agree
That I and my sailor boy married should be
But when he comes home I will greet him with joy
And I'll take to my bosom my dear sailor boy