Travelled to Ireland in Jan with a then-19 month old. It was awesome. We did Melb-Dubai (via one hour stop in Singapore but DS was asleep for that!), Dubai-London, then short flight to Dublin. We borrowed a car seat from family for the car we hired, but the hire cost for a seat wasn't very high anyway. In London we used PT the whole time (well, when we weren't walking- weather was absolutely dreamy and we were only in Earl's Court, so walked home a few times, there for 5 days) and it was great. In Dubai we had a courtesy shuttle bus, and strapped DS into the back bench seat - there was no provision for children, tsk tsk, but then again, the traffic was horrendous there and speed not an issue!
I have to say that breastfeeding is the bees knees for travelling with a child. DS was no problem on the flights because we were almost permanently attached and I didn't really give a toss what anyone thought of me feeding an 'older child', either on the planes, in Dubai, in London or Ireland. And I never got grief for it
We co-sleep, too, so we didn't need to bring any other paraphanalia than our Ergo and original Phil&Ted's Kiwi Explorer (tubular steel, if anything's going to survive baggage handlers, these will!!).
I admit that I was dreading the flights, but the real problem was keeping DS happy in the car, travelling across Ireland. He hated getting in the car (from being a real car-kid at home) after the first week, but we had to drive from Dublin to Wexford (for a wedding) to Waterford, Waterford to Kerry (after car swap at Cork airport after the hire company changed our car to a far-too-small number last minute), then Kerry to Cork, Cork to Kildare all in a week and a half. And that's considering that I cancelled out planned trip to Clare and Galway from Kerry! So, maybe train travel and staying in centres with coach tours handy is a more child-friendly option in Ireland if it's a short stay, or else plan a longer stay (I'd suggest 6 weeks to see everything and be fair on the child for travel times).
Our Bob the Builder love affair is Ireland's fault. There is so much roadwork going on there that he came away saying his first word that wasn't 'mummy' and 'daddy' - 'digger'. Got home and he was excited by all things construction!
Heaps to do in London with children. If it's nice weather, you have all those parks with squirrels, museums for older children, the historical sites, the Eye...no need to be scared to travel there with kids
Pack a bag of activities for the plane cos the airline toys are always great. If you have a laptop, bring that for portable DVD purposes and basic photo management, emailing and web surfing where there's wireless available.
that's all I can think of as a brainstorm.