This mathematic slogan is used by the Irish republican movement and by supporters of an unified Ireland. It expresses the wish and struggle to unite Ireland once again. It is often seen on t-shirts or bumper stickers.
"26" refers to the 26 counties that form the Irish Republic.
"6" means the 6 remaining counties (Fermanagh, Tyrone, Derry, Antrim, Down, Armagh) in Northern Ireland, which belong to Britain.
"1" finally represents the united Ireland as one Republic.
-"Ya see that "26+6=1"-sticker on that van?"
-"That's the car of Mrs. O'Hare, her husband sat in Long Kesh for two years."