Well, for starters, you of course would be right if you guessed that the primary role of the Marine Corps during the Civil War was serving aboard ships of the US Navy. Because that was their main job, and it wasn't for a few more decades that we begin to see the independent deployment in large numbers that characterized their role in the 20th century. However, that isn't to say that the Marines did nothing on land, because they did pitch in here and there.

For starters, the Marine Corps was tiny when war broke out, composed of 63 officers and 1,758 enlisted men, and 20 officers chose to resign their commissions, mainly to head south. And even though the Corps more than doubled its size to 3,882 by the end of the war, well, thats still less than 4,000 men total. So the point is, the Marine Corps was very small, and incapable of expanding beyond their then intended role

There are a few actions that the Marines were involved in, one of the most famous technically not being during the war. That one is the John Brown affair, where 88 Marine guards at the DC Navy Yard were quickly dispatched by rail to Harpers Ferry, where they were led by Army Col. Robert E. Lee to subdue the insurrectionists.

When war broke out a few years later, and the Union host marched into Virginia for what was expected to be a quick victory, a battalion of Marines accompanied the Army towards Bull Run. Numbering about 350 Marines previously assigned to the Navy Yards, and almost all recent recruits who had been given minimal training, the Marines didn't really distinguish themselves. Ill led, they did their best, and fought a respectable rear guard, but they eventually broke and vacated the field like the rest of the Union force, leaving 8 dead on the field, 16 wounded, and another 16 missing. Some of the recruits flat out fled in terror, and about 70 or so Marines were arrested by Army provosts as they fled back to DC.

And thats about it for notable landlocked operations by the Marines. They were heavily involved in Naval operations, and won praise for numerous actions there, such as the nighttime raid on the blockade runner Judah, as well as involvement in amphibious operations like the attack on Ft. Fisher. At wars end the Marines had had 102 killed in action, and just over twice that number died of disease.

While it hardly is an exhaustive history, American Civil War Marines 1861-65 by Ron Field is an OK overview of the Marine's roll in the war if you are looking for more.