Here's an alternative set based on the tune & chords printed in "Folksongs and Ballds Popular in Ireland Vol 3"
with the odd G7th added in the verse and an Em and Am in the chorus

THE ROSE of ALLENDALE

[G] The morn was [G7] fair the [C] skies were [G] clear,
no breath came o'er the [D] sea,
When [G] Mary [G7] left her [C] highland [G] cot,
and [C] wandered [D] forth with [G] me,
Tho' [D] flowers decked the [G] mountainside
and [C] fragrance [G] filled the [D] vale,
By [G] far the [G7] sweetest [C] flower [G] there,
was the [C] Rose of [D] Allen- [G] -dale
Twas the Rose of Allen- [Em] -dale,
the [Am] Rose of Allen- [D] -dale
By [C] far the sweetest [G] flower there,
was the [C] Rose of [D7] Allen- [G] -dale