The current entry at The Traditional Ballad Index for SKIBBEREEN says "Earliest Date: 1925 (Hayward-Ulster)." However, the following book is a bit older, and, as the title implies, the song is older still. Note that it refers to "the days of gloomy Ninety-eight" instead of '48, which seems rather strange.

From Flying Cloud: And One Hundred and Fifty Other Old Time Songs and Ballads of Outdoor Men, Sailors, Lumber Jacks, Soldiers, Men of the Great Lakes, Railroadmen, Miners, etc. by Michael Cassius Dean (Virginia, Minn.: The Quickprint, 1922), page 22:


SKIBBEREEN

Father, dear, I often hear you speak of Erin's Isle.
It seems so bright and beautiful, so rich and rare the soil.
You say it is a bounteous land wherein a prince might dwell.
Then why did you abandon it? The reason to me tell.

My son, I loved my native land with favor and with pride:
Her peaceful groves, her mountains rude, her valleys green and wide.
It was there I lived in manhood's prime and sported when a boy.
The Shamrock and Shillalah was my constant boast and joy.

But lo! a blight came o'er my crops. My sheep and cattle died.
The rent ran due; the taxes, too, I ne'er could have supplied.
The landlord turned me from the cot where born had I been,
And that, my boy, is the reason why I left old Skibbereen.

It is well do I remember that dark November day,
When the landlord and the sheriff came to drive us all away.
They set the roof a-blazing with a demon yell of Spleen,
And when it fell the crash was heard all over Skibbereen.

Your mother, too, God rest her soul, fell on the snowy ground
And fainted in her anguish at the desolation around.
She ne'er recovered, but passed away from life to Malchasene,
And found a grave of quiet rest in poor old Skibbereen.

Then sadly I recall the days of gloomy Ninety-eight.
I rose in vengeance with the boys to battle again' fate.
We were hunted through the mountains as traitors to the queen,
And that, my boy, is the reason why I left old Skibbereen.

You then, my son, was scare three years old and feeble was your frame.
I would not leave you with my friends. You bore my Father's name.
I wrapped you in my kosamane, at dead of night unseen.
I hove a sigh and bade good-bye to poor old Skibbereen.

Then, father, father! when the day for vengeance they will call,
When Irishmen o'er field and fin will rally one and all,
I will be the man to lead the band beneath the flag so green,
While loud on high we will raise the cry, "Revenge for Skibbereen!"