There's a great garage on the way into Mountrath on the Dublin side before the bend in the road into the town. It has a fab deli, toilets and showers! Don't know how long it will stay that way but it was spotless the last time we stopped.
This has now become my stop of choice on the way home via Limerick, beats the socks off McDonalds in Roscrea. Incidentally there are also toilets in the Tesco shopping centre in Roscrea if you care to detour from the N7 for a few mins.