Picturesque religious ruins are a thing that Ireland does very well. To be fair, the ruin part can mainly be attributed to some sturdy work by the Vikings and English but the Irish are keeping them picturesque.
One of Ireland's many fantastic examples can be found at Clonmacnoise, not far from the main Dublin-Galway road. Clonmacnoise was founded by St Ciaran in the 5th century at the crossroads of the old east-south road and the River Shannon. In present day middle-of-nowhere stands a well-preserved round tower, ruins of a number of churches from the 10th-13th century, cathedral, and a graveyard.
A small visitor centre also houses some really impressively ornate Celtic Crosses. I'd say some of the finest but they have a lot of competition.
Clonmacnoise's handy location makes for a very easy visit to break up the monotony of the motorway and for many will be the first taste of the Irish countryside. Even Pope John Paul II became possibly the first famous person (hopefully not the last) to deliberately visit County Offaly whilst on a papal visit in 1979.
This being said, it's probably not a standalone day trip but a pleasant hour away from civilisation (*absence of coach parties cannot be guaranteed).