The 28th Massachusetts served as a member of Kelly’s “Irish” Brigade )as evidences by the Irish Harp and Celtic writing present on the monument) in Caldwell’s Division of the Second Corps, Army of the Potomac, A Fighting 300 Regiment. The volunteer regiment was commanded by Colonel Richard Byrnes (1833 (or 34)-1864), an Irish-American officer in the United States Army, who rose to command the Union Army's Irish Brigade during the American Civil War. Byrnes was born in County Cavan, Ireland, and emigrated to New York in 1844. At at Chancellorsville and Gettysburg, his regiment, like other regiments of his brigade, had almost been wiped out so he was sent back to Massachusetts to recruit more soldiers. Two weeks after assuming command, on June 3, 1864, Colonel Byrnes fell, mortally wounded, while leading the brigade at the attack on the entrenchments at Cold Harbor, Virginia. His remains were sent to New York and buried in Calvary Cemetery. Under Byrnes' command, the regiment brought 265 men to the field, and among them, 8 were killed, 57 were wounded and 35 went missing.
The 28th Massachusetts Infantry Monument is located on Sickles Avenue, on the right or east side when traveling north and situated at the mouth or very east of the very obvious loop on Stony Hill. The monument is off road so you have to walk a few dozen feet to get to it as it is at the woodline. The monument does not face the road, but instead, faces west. 
Parking is available at small, cutout shoulders along the road, some wide, some narrow. Be sure to stay off the grass or you will be ticketed by park police. I visited this monument on Thursday, July 5, 2012 at 2:41 P.M. I was at an elevation of 571 feet, ASL. I used a Canon PowerShot 14.1 Megapixel, SX210 IS digital camera for the photos.
The Draw the Sword site helped out by the NPS narrative, the SIRIS site and my own observations offers the following description: Three foot high granite shaft with a flat top and topped with a sculptured marble eagle and set on a five foot square rough hewn base. The shaft has scotia molding and polished panels with inscriptions: A main inscription on the front, writing around the base indicating the regiments order of battle and sculptors names inscribed on the revere side, at the bottom left with the word Boston next to it. Overall height is 13.4 foot. The face of each side of the shaft, at the top, is decorated with the Shield of the United States (front), an Irish harp or the Harp of Ireland (left side), the Coat of Arms of the State of Massachusetts (person with bow and arrows) (right side) and the trefoil symbol (clover) of the 2nd Corps (rear face). The supposed flags of the Irish Brigade and the U.S. flag crossed is on the reverse face in relief. It is not apparent from looking at the sculpture that these are necessarily those flags but it is assumed I guess. At the top of the monument, front side, just under the crown is inscribed Faugh A Ballaugh, which is Gaelic for Clear the Way. The monument cost $500.00. Flanking markers have gable tops with inscriptions on the west faces, 1.6×8 foot. The monument marks the position attained by the 28th Massachusetts regiment about 5:30 on July 2, 1863 as they engaged the Confederates.
The monument was erected in 1885 by the Survivors and Friends of the 28th Massachusetts Regiment. I could not recover the exact date. The monument is composed of: Monument: granite; Eagle: marble; Base: stone. The monument's dimensions are: Overall: approx. 13 feet 4 inches x 5 feet x 5 feet. The monument was sculpted by T. M. Brady and A. Ford from Boston (as evidenced on the reverse side of the monument). On the front side of the monument is the main inscription and around the base is inscribed the order of battle, all of which read:
(Around Base):28th Mass.
2nd Brig. 1st Div.
2nd Corps.
(Front):Faugh A Ballaugh
28th Mass. Inf'y
Vol.
Col. Richard Byrns Com.
Meagher's Irish Brig.
Col. Patrick Kelly. Com.
This regt. went into battle
July 2. 1863.
Numbering 220
officers and men.
101 of whom
were killed or wounded.
Erected by the survivors
and friends of the Regt.
to mark the spot where it
fought in defense of the
American Union.
The 28th Massachusetts Infantry Monument is a contributing feature to the Gettysburg National Military Park Historic District which is nationally significant under NR Criteria A, B, C & D. Areas of Significance: Military, Politics/Government, Landscape Architecture, Conservation, Archeology-Historic. Period of Significance: 1863-1938. The original National Register Nomination was approved by the Keeper March 19, 1975. An update to this nomination was approved by the Keeper on January 23, 2004. The monument is identified as structure number MN167-B.
From the Nomination Form:
1 of 25 mn in Park honoring MA commands in Gettysburg Campaign. Marks position attained by 28 MA on July 2, 1863 as they engaged the Confederates here. Placed at the Loop, on E side of Sickles Ave.
Short Physical Description:
Mn & 2 flank markers. Rough-hewn base, 5' sq. Shaft 3' sq. All 13'4" H. Shaft has scotia moulding, polished panels w/ inscriptions, flat top w/ sculptured marble eagle. Flank markers, 1'6"x8", 2'6" H. w/gable tops w/ inscriptions on W faces.
Long Physical Description:
Monument that has two flanking markers. Monument is a three foot square granite shaft with a flat top that contains a sculptured marble eagle and set on a five foot square rough hewn base. The shaft has scotia molding and polished panels with inscriptions. Overall height is 13.4 foot. Flanking markers have gable tops with inscriptions on the west faces, 1.6x8 foot. Located at/above the Loop on the east side of Sickles Avenue.
My Sources
1.
NRHP Nomination Form
2.
SIRIS
3.
Stone Sentinels
4.
Virtual Gettysburg
5.
Draw the Sword
6.
Historical Marker Database
7.
Wikipedia