I was catching up on the recording of the live event from yesterday and there was some discussion regarding Cleburne siding with the Confederacy given his Irish heritage and limited time in the states. There are some relevant passages in "Pat Cleburne Confederate General" by Elizabeth and Howell Purdue that I think are enlightening as to Cleburne's thoughts.
In a letter to his brother prior to war, Cleburne would write, "I can now stand at my office window and see a foreign nation on the other side of the river. As to my own position I hope to see the Union preserved by granting to the South the full measure of her constitutional rights. If this cannot be done I hope to see all the Southern States united in a new confederation and that we can effect a peacable separation. If both of these are denied us I am with Arkansas in weal or in woe."
A short time later he would also write his brother, "I am with the South in life or in death, in victory or defeat. I never owned a Negro and care nothing for them, but these people have been my friends and have stood up to me on all occasions. In addition to this, I believe the North is about to wage a brutal and unholy war on a people who have done them no wrong, in violation of the constitution and the fundamental principals of the government.....We propose no invasion of the North, no attack on them, and we only ask to be let alone."
In 1863 he would state in regards to the North's opposition to slavery, "It is merely the pretense to establish a sectional superiority and a more centralized form of government, and to deprive us of our rights and liberties."
Cleburne may have only spent a portion of his life in the south, but I think his writings make his allegiance quite clear and demonstrate the strength of his feelings.