Explain the quote, "no single irritant of the 1850's was more consistently galling to both sides ... "

This quote relates to the development of the Fugitive Slave Law developed during this time to attempt to appease both the North and South. It was meant to benefit the North, for example by giving them the territory of California as a "free state", a non-slave state with voting rights. And to appease the South, what became known as the Fugitive Slave Law, which obligated Northerners to return slaves who had escaped. The "irritant" is the fact that Northerners attempted to avoid repatriation and returning of the slaves, so while the South had given their concessions of territory, the North had difficulty implementing the obligatory return of slaves.