The line is in the 2nd verse of "The Star of Slane:"

In beauteous Spring, when the warblers sing,
And their carols ring through each fragrant grove;
Bright Sol did shine, which made me incline
By the river Boyne for to go to rove, I
was ruminating and meditating
And contemplating as I paced the plain,
When a charming fair, beyond compare,
Did my heart ensnare near the town of Slane.

From "Evenings in the Duffrey," Patrick Kennedy, 1875, McGlashen & Gill; from "The Golden Treasury of Irish Songs and Lyrics, vol. 2; Wilson's Tales of the Borders- These references on line; your song may be a variant. I don't know when "Star of Slane" first appeared.

"The Star of Dundalk, and other Poems," S. Lover

In beauteous spring when birds do sing,
And cheer each myrtle shade,
And shepherd swains ser'nades the plains
To find their lambs that's stray'd,

Nigh Boden's Grove I chanced to rove
To take a rural walk,
When to my sight appear'd in white
The Star of Sweet Dundalk.

Your beauteous face my wounds encrase,
And skin more white than chalk,
Makes me regret the day I met
The Star of Sweet Dundalk.

Dublin Literary Gazette, reproduced in "New Estate, or the Young Travelers, in Wales and Ireland," 1831, London, Harvey & Darton. Also found with Google, on line.