Does Brendan Gleeson also play the role of the fiddlist in the confederate field hospital?

The character at the starting of the movie, the one who is comforting the dying soldier at the field hospital and the one appearing at a later time as Ruby's father is the same one.

Both are Brendan Gleeson playing Stobrod Thewes.

From the Wiki:

Three years later, Inman fights in the Battle of the Crater and survives; he then comforts a dying acquaintance from Cold Mountain, while fellow soldier Stobrod Thewes plays a tune on his fiddle.

Also this explains:

By the time the story starts, he's gone off to fight in the war, and Ruby thinks he may be dead. Stobrod turns up partway through the story, having deserted the army......Eventually, the Home Guard finds Stobrod and Pangle and brutally shoots them. Astonishingly, Stobrod survives this....

Also if you are still in doubt here's movie script by the director. A search for Stobrod Thewes and we get him at the beginning of the movie:

Inman turns away, sees another Rebel, extravagantly costumed, a strange FIDDLE head protruding from his knapsack. This is STOBROD THEWES. He's bent over a dead Federal, examining his mouth. He reaches behind his back and roots around in the knapsack, producing A PAIR OF PLIERS, WHICH HE INSERTS INTO THE CORPSE'S MOUTH. He's yanking away when A SWINGING BOOT connects with his head and knocks him to the ground. ......

INT. FIELD HOSPITAL. NIGHT

               Stobrod stands over Oakley. Consults with Inman.

                                     STOBROD
                         What about Bonaparte's Retreat? That's 
                         one I play.

                                     OAKLEY
                         Play me something sweet. Like a girl's 
                         waiting for me.

So, he IS Ruby's father at the beginning of the movie, with whom you are having doubt.