Drawing on Kayaking experience I reckon that at high Water this River could be excellent for SUP. Two Reasons. Firstly, there are plenty of rapids as oppose to weirs and Secondly at the right level the Standing Wave could be retentive and playful.
This extract is from irishwhitewater;
The Inny is one of the most popular grade 2 rivers in Ireland, this is due to the ease of access to its put in and take out, and also due to its central location in the country. It provides a pleasant days paddling or an introduction to moving water, with plenty of short rapids to entertain.
The river is also known for the Meat Factory Hole/Wave on high water. Downriver from Ballymahon Bridge an excellent wave-hole forms on higher water. It stretches almost entire width of river and forms a brilliant wave that is retentive enough to stay in as long as you want but you don't need to worry if you mess up as it will push you out quite quickly. All types of freestyle moves are possible on this wave.
River level gauge
If there are small waves under the bridge in the town it should be worth paddling. If the ledge on the bridge is covered the meat hole should be at a perfect level. Also at Shrule Bridge, a water level gauge is visible just upstream of the bridge.
I am planning to paddle this some Saturday or Sunday morning and looking for at least two paddlers with reasonable experience in Surf to take this on with