Why is the Star of the County Down referred to as Colleen?

Question

While listening to the Irish Rovers version of the traditional Irish song "Star of the County Down", I noticed an apparent discrepancy in her name:

Near Banbridge town, in the County Down
One morning in last July
Down a boreen green came a sweet Colleen
And she smiled as she passed me by.

[...]

As she onward sped, sure I scratched me head
And I looked with a feeling rare.
And I said, says I, to a passerby
"Who's the maid with the nut brown hair?"

Well, he looked at me, and he said to me,
"That's the gem of Ireland's crown.
Young Rosie McCann from the banks of the Bann
She's the star of the County Down."

If her name is Rosie McCann, then why does he keep calling her Colleen (including in the chorus of the song)? For that matter, why does he call her Colleen before even knowing who she is, as he needs to ask a passerby? Was Colleen in Ireland just a "placeholder" name, to be used for someone if you don't know their actual name, like Jimmy in Glasgow or Charlie in Ghana?

Answer 1

Colleen is the Anglicised version of the Irish Gaelic cailín, meaning young woman or maid.

Derivation: caile (“maid”) +‎ -ín (diminutive suffix)

Although girls are sometimes named Colleen, in the context of the lyric it shouldn't have a capital C.

The New English-Irish dictionary has other usages.

Answer 2

From the Wikipedia entry for Colleen:

Colleen is a common English language name of Irish-American origin and a generic term for Irish women or girls, from the Irish cailín 'unmarried girl/woman', the diminutive of caile 'woman, countrywoman'.

(My emphasis)