My parents gave me a very normal English name with a very abnormal spelling. I changed it to the standard spelling by deed poll I was so fed up of people not being able to spell it.
If you give a child a name with a spelling that is not standard in the country they are living they will spend their lives spelling the name.
Also the names are going to be pronounced more by English people than Irish, so might not get the sound right. Would that bother you?
Not through lack of trying. I've seen a few posts about Niamh and Neve, to me they sound exactly the same and I'm normally good at picking up subtle sounds.
Where in the UK will you be living, I'm related to a Niamh who has lived her entire life in Yorkshire and it sounds different with that accent, much deeper, more knee-ev. Are you going to be happy with that? Well with whatever accent your children will grow up with.
Then there is teaching your child to write their own name. I went to school with a Philomena, she thought she was stupid because at 5 she couldn't write her name when all the Traceys and Janes could.
No matter how beautiful a name is in Irish, is it as beautiful pronounced with an English accent?
I suppose it depends how much you are attached to the spelling, how important you feel it is.
Siobhan, Niamh and Sinaed are common enough for English people to know the name and the spelling, although I know there should be accents over the letters but not where.
Although I did once shout Siobhan X to a crowded waiting room and Siobhan and her mother looked at each other stunned. In 15 years no one had pronounced her name correctly without them explaining it.
I was also at school with a Dymphna which can easily be worked out with phonetic English spelling.