Hi, I’m a teacher (now working as a private tutor) and I wouldn't worry too much about either of them being behind at 5 and 7. Assuming they’ve had plenty of play based learning in Ireland, they will quickly catch up and possibly overtake their peers here in England who may have been pushed into formal learning too soon anyway.
The one thing that might throw your children is the English government led absolute focus in schools now to teach reading almost entirely by relying on synthetic phonics. Synthetic phonics are taught rigidly and systematically in most schools here now and children as young as 5 are tested on phonic knowledge regularly. If Ireland doesn’t teach phonics in the same way then your children might struggle with some of the sounds and terminology. For example, most Y1 children here know what a split digraph is so if Irish schools don’t teach like that it might take them a little while to learn these things.
If you think that might be a problem, let me know and I can suggest some books and videos you could start using now to get them up to speed in phonics. Btw, I’m definitely not suggesting I agree with the ‘phonics first and foremost’ stance of the British government - personally I think it puts a lot of children off reading and makes it a laborious exercise with little understanding or enjoyment. Therefore, I’d also suggest you spend the next few months doing whatever you can to encourage a love of reading… read to them daily etc etc.
In one of the schools I worked in, I regularly taught children who arrived at the school midway through with little to no English. It usually took around 6 months for them to catch up in areas of the curriculum they’d not covered before and that was when they were also having to learn a second language.
It’s easy for me to say I know, but try to relax and they’ll be fine. They are young enough to adapt and have plenty of time to catch up if necessary. They will obviously need a bit of time to settle in and, if I were you, I’d focus on helping them make friends and be happy at school first - then the rest will follow.