Zimmermann's 'Songs of Irish Rebellion' quotes these verses from a broadside "Ireland Emancipated" celebrating the Emancipation Act of 1829:-

"Six millions of brave Catholics
Will support great George's crown..."

According to the Roud Broadside Index this comes from a broadside in the Madden collection printed in Dublin and beginning "Rejoice you sons of freedom of each religious creed".

Another song in Zimmermann is 'The Banished Defender', sometimes also known as 'The Brave Defender', which begins "Poor Catholics of Erin give ear unto these lines I write". I've never heard anyone sing this, and the fifth verse seems to me to be the nearest I've ever seen to being completely unsingable (now there's a challenge!!!)

"Transubstantiation is the faith that we depend upon;
Look and you will find it in the sixth chapter of St John,
When Moses and Elias they told us of our Heavenly Church,
That we in future ages should suffer persecution much."

I think steves is probably right to locate the song or tune 'Brave Catholics, We Commend Thee' in early 19th century Ireland, and I think there is probably a connection to Daniel O'Connell and Emancipation, but I can't trace anything closer than the above examples.

Matthew