Ireland is a magical place, you will love it! As I read somewhere in a photo magazine recently, every place in Ireland has a story...and a photo taking opportunity.
Be prepared for very changeable weather. I've never been there in the summer but it rains a lot, though in spurts, so the weather should not stop you from picture-taking. Make sure you bring stuff to protect your gear.
Take plenty of pictures and, in difficult lighting situations, bracket. I used matrix metering and spot metering. Expose for the bright areas, you can always pull detail out of darker areas with image editing software.
Don't forget an electricity converter or plug adapter for your battery charger. If your charger is like mine, I only needed a plug adapter which I purchased online at Radio Shack (I used their 800 #, though, for some help. There are so many selections and it got a bit confusing without some guidance).
Don't forget to bring a tripod and, yes, bring an external flash and your telephoto lens extension.
Enjoy your trip. Bon Voyage!
Related reading:Auto exposure bracketinghttp://www.digicamhelp.com/what-is-exposure-compensation/auto-exposure-bracketing.htmDigital cameras, accessories and airport x-rayshttp://www.digicamhelp.com/x-ray-and-photo-equipment/airport-x-ray.htm Stained glass windows in cathedralshttp://www.digicamhelp.com/stained-glass-photography/index.htm Note: center-weighted metering is recommended for photographing stained glass