Update
Took him to the Thai Garden in Blanchardstown village. Really nice and he found something he could eat. We did ask the waiter re "without peppers" but he either didn't understand or knew that there were certain things they couldn't take them out of. I got the impression he could have had what my brother had - it had peppers floating in it - as a special request, but the waiter wasn't going there. He was very happy with what he had, as we all were, excellent value at E150 for 4 including wine and 3 courses for all of us.
Dad would go back! Result!
Lemongrass in Maynooth is also on list for trying.