Greetings from Ireland,
You're going to love this Heart of Ireland tour.
I enjoyed a decade of happy years guiding over 100 of these for Rick Steves before I retired from RS guiding last season.
To answer your questions:
1. Shannon airport is tiny and getting from there to Ennis with Bus Éireann is easy and cost-effective. A taxi wouldnt be incredibly expensive either and its a 25 minute drive, this might be worth considering if you like to be really hassle-free. (In my experience well over half of the Tour Members for this Heart of Ireland tour still landed in Dublin airport and took the train or bus across to Ennis to begin the tour)
2. Ennis is a lovely town with warm friendly people, it has a rich history but these days its a little underwhelming in terms of things to do. You're absolutely right to go and visit the Friary, although try and join a guided tour so that you dont miss any of the interesting details to be found there. Theres a few reasonably good pubs and cafes, and of course good trad music in the later evenings. The Poets Corner in the hotel bar has a decent session most evenings. Saturday nights are very lively in Ennis, most pubs are absolutely packed. On my 100 visits to Ennis, my experience was by Sunday evening it was usually a bit deserted and very quiet. Unless Clare were playing in the hurling or football, and then it was an entirely different affair :) .... (Easter Sunday evening might be a bit more lively than a normal Sunday).
3. Your tour hotel, the Old Ground (which is a nice spot with friendly staff), is right across the street from the Cathedral, so thats handy for Easter Mass. Theres no dress attire for Mass, so wear what makes you comfortable and/or you feel is respectful. Certainly no need for a suit or tie. Some of the older generations tend to still dress more formally, but the small percentage of the younger generations who choose to attend Mass these days mostly dress casually.
4. Theres so many great things to see and do in Dublin during your free time, your Rick Steves guide will be happy to assist and advise also during the tour, but from my experience about 5 or 6 of the 28 tour members would always visit the Guinness storehouse in their free time and they enjoyed it despite the often overwhelming crowds that have become the norm (the Guinness storehouse used to be on this itinerary but got dropped after a year or 2.)
Jameson was popular to a lesser extent, and many more tour members spoke highly of visiting the Epic Irish emigration museum (highly rated by Rick).
Your walking tour of Dublin will include the Book of Kells and Trinity college and usually includes the courtyard of Dublin castle, so you'll get a good overview of the city and its history.
In 100 tours I never once met a tour member who went to a Bohemians or St. Patricks Athletic soccer match, so thats cool that you are considering it. Certainly would be a unique and authentic experience for you (excuse my bias but I hope one of them is playing Derry City that night and lose heavily). These soccer or football games (confusingly some of us Irish use the term 'soccer', some use 'football', some even use both references in the same conversation!) usually attract around 5,000 fans and theres a decent buzz in the small stadium. A Gaelic football or Hurling game would in my opinion be an even more unique cultural experience, and the opening games of the championship seem to be starting on the weekend your tour arrives into Dublin. If Dublin are playing, then Id highly recommend trying to check it out. I brought Rick Steves to Croke park some years ago to cheer on Donegal and he loved it, such a fun day, Rick even attempted a pitch invasion!
Heres the fixture and game information:
https://www.gaa.ie/article/gaa-master-fixtures-schedule-for-2026
Enjoy your Heart of Ireland tour, youll have a great time. I'm jealous :)
Stephen McPhilemy
Rick Steves Ireland Guide (retired) of 20 years