Irminsul is a Germanic neo-pagan  
Its name probably meant "mighty pillar" in Old Saxon. The name originates from Old Saxony (roughly corresponding to present-day Lower Saxony), where it was denoting a wooden pillar, or a simple tree trunk, raised in the open air at a place of worship. While all the original objects were destroyed with the spread of Christianity and little is known about their exact appearance, modern symbol is generally based on the Externsteine relief, a depiction of the Descent from the Cross scene which is carved into the side of the Externsteine rock formation, situated near Detmold, Germany. The scene, generally thought to be dating from 12th Century, contains a symbol which vaguely resembles a bent palm tree and is sometimes explained as such. 
This symbol was first described as an irminsul in 1929 by archaeologist Wilhelm Teudt, who believed that the Externsteine site was a pre-Christian place of worship centered around an irminsul and that the relief was created in order to commemorate the victory of Christianity over paganism. 

A popular version of the flag with irminsul depicts the white symbol within a white ring, all on black field.
https://www.crwflags.com/fotw/flags/qt-z_sym.html
Tomislav Todorović