On the other hand I would go via Roscrea which despite the super new Tesco store has ancient and religious roots and well worth an hour or more. Then up to Birr for a look at the Castle grounds and telescope. Pub lunch and the country route to Tullamore (nice new rd) for the Herritage centre. then on to Trim.
Combine both routes and drive over the Slieve Bloom from Birr to Portlaoise if yo want really scenic. You could race through in under 3 hours but then you could easily take a couple of days..
Thank you both very much