When a cheerful Irish love song (or indeed something suitable for a wedding) is required, I usually go for "Sweet Forget Me Not". It's an Irish parlour song rather than a traditional song per se but is well known on the Irish scene. De Dannan recorded it.
Helen


The Sweet Forget-Me-Not

Fancy brings a thought to mind
Of a flower that's bright and fair
Its grace and beauty both combine
A brighter jewel more rare
Just like a maiden that I know
Who shared my happy lot
She whispered when we parted last
"Oh, you'll forget me not"

    She's graceful and she's charming
      Like the lily in the pond
    Time is flying swiftly by
      Of her I am so fond
    The roses and the daisies
      Are blooming 'round the spot
    Where we parted, when she whispered
      "You'll forget me not"

We met, I really don't know where
But still it's just the same
For love grows in the city streets
As well as in the lane
I gently clasped her tiny hand
One glance at me she shot
She dropped her flower, I picked it up
'Twas the sweet forget-me-not

And then there came a happy time
When something that I said
Caused her lips to murmur "Yes"
And shortly we were wed
There is a cott' down in the land
And a tiny plot
Where grows a flower, I know it well
It's the sweet forget-me-not