From the Super Mario Wiki
The Pi'illo Ring is an item found exclusively in Mario & Luigi: Dream Team, and the eighth and final item collected during a series of item trades pertaining to a sidequest in Wakeport. Mario and Luigi find this item buried in the grass near the twin fountains of northeast Wakeport after a Toad points out its location following their returning of his Lost Camera.
Mario and Luigi must then head back to the same Hooski who gave them the Plush Zeekeeper earlier, and he will use the ring to propose to his love interest. The proposal is successful, and the now married Hooski thanks the Bros. by giving them a 1-Up Mushroom, a Heart Bean DX, Rally Wear, and a Sap Hammer.