I. To determine the recommended phase 2 dose (RP2D) of intratumoral TLR9 agonist SD-101 (SD-101) in combination with ibrutinib and radiation in patients with relapsed or refractory low-grade follicular lymphoma. (Phase Ib) II. To determine the safety and tolerability of SD-101 in combination with ibrutinib and radiation in patients with relapsed or refractory low-grade follicular lymphoma. (Phase Ib) III. To evaluate the efficacy of intratumoral SD-101 in combination with ibrutinib and radiation in patients with relapsed or refractory low-grade follicular lymphoma by assessing overall response rate. (Phase II)