Ireland and ?!?

Paris-and drive to Chartres, spend the night if it's when they have the summer light show, then drive to one or more Chateaux.

Soul-searching time. How much does DH want to do "another country"? And what exactly does that mean?

You could easily just add to your Ireland itinerary (more Ireland, go slower, etc.) - plenty to see there.
Yes, Scotland would be an obvious and easy bolt-on. But of course you're not really going to see "the country" in just a few extra days - you can get a taste of Scotland, and maybe that's OK. OTOH, if you're looking for a DIFFERENT "other country" (that is, a contrast to Ireland), Scotland is gonna seem pretty similar (speaking grossly, as an outsider - no offense to Irish and Scotsmen everywhere).
The above aside, Scotland makes a lot of sense. It's quick and easy to get to (I'd fly). It's beautiful, has plenty of charm, and is fun. Rich material for a photographer, and really awesome pubs (with music!). Check, check, check...an easy call, as long as you can make peace with the fact that you can't possible see even just the highlights with the time you have. Accept that, promise you will return another time to take a deeper plunge, and you will have a fine time.

Or...if you really want a contrast...with 5-6 nights, get a cheap flight to Marrakech and embrace the differences!

Quite frankly, if this is your first time in Ireland, I'd suggest staying the 15 nights to really see a good chunk of Ireland. With that amount of time, you can do a good loop around the country and do stuff like spend time driving around Ring of Kerry, Beara Peninsula, side trips to Skellig Michael, overnight on Aran Islands instead of just a day trip.

Cross the border and head into Northern Ireland and spend time up there - Belfast, Derry, Antrim Coast. Most people just spend their time in Dublin and a couple other spots in Ireland and don't think about taking the time to head into Northern Ireland.

If you feel like you must see another country and Scotland might take a little too long to get to where you want to go, then maybe consider taking the super fast ferries from Dublin to Wales and spending some time there. The ferries only take a little over 3 hours.

I've done something similar, Paris is a great choice. The first time I took my teens to Europe we spent time in the UK and then flew to Barcelona for 5 days. It was late winter so we appreciated the warmer weather but the real plus was a totally different cultural experience, scenery, food etc. Paris? Barcelona? Sevilla? Lisbon? I would just look at non stop flights 2-3 hours max to minimize travel time and these cities also have lots of direct fights back to the east coast.

I am currently planning a two week Ireland itinerary for later this year and two weeks is not that much, there is so much to see in this country even though it's small. My wife and I did an Ireland trip in 2002, with five night in Dublin (too much, we did that because we had been in London previously and four nights there was not enough on a first visit. Note to self: Dublin <> London). We hopped an Aer Lingus flight to Edinburgh for three days, and loved that city. Then six days in smaller towns and countryside in Ireland. As much as Edinburgh was great, we really shortchanged Ireland with too much time in Dublin and the side trip to Scotland. This year we will do it right, but I still have a hard time fitting everything I want to see in in two weeks. And that's just Republic of Ireland, not even venturing into Northern Ireland. These days I try to resist the temptation to tack another country onto a trip, otherwise you leave feeling like you had an inadequate visit.

I think you need to consider that there are only a few direct flight options from Shannon. Berlin, London, Krackow, Paris, Edinbourgh, Warsaw and Zurich. Dont want to spend an entire day changing flight etc. You mentioned you want to do France "in a few years." Why not see Paris this trip? 10 days Ireland. 5 days Paris. Serenity of Eire and the amazement of the City of Lights. One compliments the other. Pack light, book now!!!

Another vote for Scotland! I don't find it all that similar to Ireland (and I have been to Ireland four times). I was only able to spend four nights, all in the Glasgow/Loch Lomond area, but it was wonderful. You could see a lot in your time frame if you fly into Edinburgh.

Agree with Ceidleh. Consider spending the entire time in Ireland and Northern Ireland. They are both marvelous places, very beautiful, with a good mix of cities and county side. Technically Northern Ireland is another country and since they border each other you can do two and still not lose time going from Ireland to Scotland or ??

I'm for Scotland. Fly to Edinburgh and pick up a car, drive to Inverness, then to Skye, then spend a couple of days seeing the highands - Fort William, Glencoe, Loch Lomond and Stirling, drop the car back in Edinburgh, and spend the last day and night there before flying home. If you can eke out 6 nights for Scotland that would be better but even 5 nights would be well worth it. I've been to Ireland twice and Scotland once and they really are quite different.

I agree that spending all of your 15 nights in Ireland is a good idea, especially if you decide to spend time in Northern Ireland as well as the Republic. On our first trip we had 15 nights and managed to cover the entire island. We spent three nights in Belfast and drove through Counties Antrim and Derry. We enjoyed our time in Northern Ireland and found it to have a somewhat different vibe than the Republic. The currency is different, the accents are different, the road signs are different (mph rather than km--though sadly they also drive on the left side of the road) and really, it had a much more British feel to it. We experienced it as another country (which it is!) That being said, on our recent return trip to Ireland we had 24 days to work with and decided to add Scotland to the itinerary. We really wanted to spend a full three weeks exploring the south and west of the Republic of Ireland (the scenery is fabulous there--your husband should have many opportunities for excellent photo shoots), so we limited ourselves to three days in Edinburgh. It's a really interesting city and there is a lot to see. We considered a day trip to St. Andrews but decided it might be too much given our time frame and jet lag, but 5 nights at the end of your trip would give you a better opportunity to do one or two day trips. Our route was the opposite of yours--we flew into Edinburgh and out of Shannon. We found an inexpensive fare from Edinburgh to Cork on Stobart Airlines so travel between countries was pretty easy. What ever you decide, you're going to have a wonderful trip!

I'd vote for either Scotland (focus on the Highlands or the Islands) or London. I'd make the decision on how you time in Ireland is used. If you are mostly doing countryside, then I'd go for London. In 5-6 days you could get a nice taste of London and do 1 or 2 day trips by train (Windsor, Canterbury, Dover, Winchester, Bath, Cardiff Wales, York, Stratford-upon-Avon, and others are all good possibilities). And flying home from London should be fairly easy. There's LOTS to see and do there- arts, history, and science museums (and other kinds, too), mostly free, The Tower of London, Kew Gardens (and others), The British Museum, THEATRE!!!, and much more.

While I understand why everyone is saying to spend the whole trip in Ireland, I also understand your desire for a "change of venue" to see something different on this trip.

While Shannon Airport has a limited selection of European destinations, it's not too far to get to Dublin Airport, which, as a Ryanair hub, has flights all over Europe. Here's a list from Wikipedia - happy choosing! https://en.wikipedia.org/wiki/Dublin_Airport#Airlines_and_destinations

Adding time to do Ireland in more breadth &/or depth is a good option. Since your husband likes to see and do more, this would meet his needs. The photo options are unlimited. We have been there in 1996, 2011 & 2015 usually for 10-12 days each time. We always drive on our own to be free to move about. I would recommend Scotland as a different trip altogether.

Someone also mentioned Barcelona. It would be perfect for a 5 day addition, as would Paris (however you said a future trip to France is in the plans). Barcelona is a beautiful city with much to offer in the way of food, culture, architecture, art, etc. We were introduced to Barcelona on two occasions, but only for a day or two on earlier trips. In 2014 as part of a trip to Spain, we stayed in Barcelona for 5 nights. It is one of our favorite places. Trip Advisor, sorry RS, will give you a good look at "things to do" there. Barcelona would also give you an introduction to Spain for future reference.

I just wanted to thank everyone for all of their wonderful suggestions! Every place that was named sound amazing and I hope to visit them all at some point :)

Right now we are leaning towards Scotland due to the (relative) proximity and cost.

Thanks again for all the suggestions!

Not that it's that big a deal, but if you visit Scotland, you'll need to switch from Euros to Pounds Sterling. Of course, visiting both the Republic of Ireland and Northern Ireland means the same thing. Happy travels.