Today is the Feast of St. Martin de Porres, a Peruvian lay friar of the Order of Preachers and patron saint of, among other things, social justice and barbers.
His patronage of barbers follows from his time as a barber. His patronage of social justice follows from his personal experiences; his mother was a former slave, his father a Spanish nobleman who ignobly abandoned her and their two young children, and within his Dominican convent he also experienced class- and race-based bigotry.
St. Martin himself, from what I've been told, wasn't particularly occupied with issues of social justice as such. The virtue that consumed his life was charity. He sought out lowly positions from which he could serve others in love, and spent thirty-five years as infirmarian, caring for (and often miraculously healing) the sick of his convent and his city.
One story is told of a time when an epidemic was raging through Lima. Brother Martin brought those he met who were sick to the convent, until his prior ordered him to stop, lest all the friars in the place get sick. When he then found a man on the street bleeding from a knife wound, though, Br. Martin brought him to his own room to treat him. The prior scolded him for disobeying his order, and Br. Martin apologized, saying, "I did not know the precept of obedience took precedence over that of charity."
The prior then commanded him to do as he saw fit in caring for those he met.