"Visit the Twelve Bens" etc. Well you'll either see them or not depending upon if the weather is clear while you are in the area: you don't need to schedule any extra time to "visit" them if seeing views of them is the limit o fyour ambition. If you intend some hill-walking in the 12 Bens, be aware that there are not official footpaths nor signs in the range, which is mostly private land, and that all walking routes over the hills are very rough walking over steep and loose rocky terrain, exposed in places, in places. I would advise buying a book on hill-walking in Ireland, as the rights of way/footpath sitation in Ireland is very unsatisfactory and you need detailed advice on what to do and where to go, as just buying a map isn't enough. There is the "national park" interpretation centre just north of Clifden, where you pay to go in and can walk a short paved path up a little hill. OK with children, or for people who don't really do proper hill-walking.
No logistical problems difficulties climbing Croagh Patrick if that is your aim. Just go to the car park and set off from there. But the path is very eroded and steep on the upper parts - a pile of rocks which move - and it will not be an enjoyable walk if the weather results in the rocks becoming slippery. People do occasionally have really nasty accidents on this path. But I saw a completely blind lady manage it with help from a companion (on a dry day) - I was quite impressed given I nearly ended up on my bum a few times.
Slieve League is also fairly straightforward of access - you just drive to the car park along the scary narrow road and set off up the path. There is some quite exposed, steep, eroded ground, to cover if you intend to go right to the high point. You can arrange it as a circular walk if you have a map, though the routes are a bit indistinct in places, and you just have to set off in the right direction over the land at times.
I think you might find Day 2 a bit much. But after you've done the Boyne valley attractions, you can see how the day is going and drive straight on to Ennis if time is pressing. Cross country from the Boyne, getting to Ennis will be a slow drive. There's nothing much in Athlone, pleasant enough but nothing special. You are right, Day 11 is a bit much. It will talk you about 4 hours or so to drive back to Dublin after you've finished.
I did a very similar holiday myself about 3 years ago, at exactly the same time of year. It worked better to take in the Boyne valley monuments on the way back from Donegal. They are more nearly on the way from Dublin to Donegal than on the way from Dublin to Clare. I would recommend trying to stay more than one day in more places and missing out some of the intermediate stops. My wife says she would have preferred to just visit Connemara and Donegal, and forget the stuff around Sligo/Westport. It was impossible getting accommodation anywhere near Donegal town, but once we headed out to Ardara and beyond there was plenty of availability.
