This phase Ib/II trial studies the side effects and best dose of toll-like receptor 9 (TLR9) agonist SD-101 when given together with ibrutinib and radiation therapy and to see how well they work in treating patients with grade 1-3a follicular lymphoma that has come back after a period of improvement or no longer responds to treatment. Immunostimulants such as TLR9 agonist SD-101 may increase the ability of the immune system to fight infection and disease. Ibrutinib may stop the growth of cancer cells by blocking some of the enzymes needed for cell growth. Radiation therapy uses high energy x-rays to kill tumor cells and shrink tumors. Giving TLR9 agonist SD-101 with ibrutinib and radiation therapy may induce an immune response and prolong anti-tumor response.