Could anyone possibly tell me about this uniform?

[MacMillan of Rathdown]

He was born around 1836, died in 1880 if that helps any... I've been searching around myself, but can't seem to find which uniform this is, or if it's a uniform of sorts at all.

If you send a copy of the photo to the Army Museum in London, along with the soldier's name, they should be able to assist you. To me it appears to be a photo dating from the period after the Crimean war. At that time the standing collars on British infantry tunics were open, not closed, in the front.

There is one other clue that helps in possibly dating the photograph. The soldier is holding a shako of the type issued between 1861 and 1868. Further, the pompom appears to be black, which would tend to indicate a light infantry company. I can't quite make out the insignia on the right sleeve, but presume it to be some sort of rank insignia.