We have investigated the ultrahigh resolution of light ion irradiation induced magnetic patterning. High aspect ratio silica masks on Co/Pt multilayers were obtained by e-beam lithography and reactive ion etching with feature sizes down to 30 nm for lines and 20 nm for dots. He+ ion irradiation of the magnetic layers through these masks was used to pattern the magnetic properties. We chose irradiation irradiation conditions such that patterned areas had a coercive force three times lower than the protected areas. After mask removal, we obtained high resolution and high density, planar magnetic nanostructures. Magnetization reversal processes in the line patterns are analyzed with numerical simulations. We suggest that this irradiation induced magnetic patterning technique is promising for both present magneto-optical storage devices and future near field recording.