The Court has always been a political body. This is really nothing but legal realism at its most vulgar. Everyone accepts that the personal political opinions of Supreme Court justices play a role in their decision making and that the Court itself is sensitive to the political impact of its decisions. That function cannot be understood apart from an understanding of the function of a written constitution. Irons quotes this passage, but I'm not sure he grasps its meaning.
Legislatures are thoroughly political bodies.
But what can be most fruitfully garnered from Justice Jackson's statement is that, when the Supreme Court exercises judicial review, it makes choices about what will be insulated from politics. The Supreme Court has, of course, given different answers to this question over the course of our history. In Irons' view, and in mine, many of these answers have been wrong, as when the Court protected the rights of slaveholders in Dred Scott , or when it granted constitutional status to the unregulated free market in Lochner , or on the numerous occasions it has refused to protect political speech from government suppression. But, over the course of the last half-century, Irons believes, and I largely agree with him, that the Court has gotten much right. The Supreme Court of this era has protected unpopular speech, racial minorities, the rights of the accused, and has granted constitutional status to rights of personal privacy.