Fill in the soil, pack it down well, and water very well. If you are planting bearded irises, their tops should be barely covered with good soil. No extra watering should be necessary. If you are planting beardless irises such as Siberian Irises or Japanese Irises, they should be two or three inches deeper. Keep beardless irises well watered for the remainder of the growing season.
Borers are a problem especially in the northeastern quadrant of the USA and nearby points.
The single most important step to take in avoiding borer problems is to do a very thorough cleanup well before the expected last frost, removing all withered foliage and debris. These were likely where eggs were laid last fall and overwintered, and taking it away before they hatch goes a long way toward eliminating problems. Beneficial nematodes have been suggested as a biological control mechanism. They need to be applied and active when the eggs hatch and the larvae are making their way to target leaves, triggered by the day that the temperature first goes above 70-72 degrees, often soon after the last expected frost date. The most effective active measure that can be taken is to spray with a systemic insecticide like Cygon-2E, a powerful chemical.