Hello Georgie
I do hope you are okay - if your DD has mixed with other children in a playgroup setting you may have already had cmv and didn't know it so you may have immunity. I hope so.
CMV is one of the torch infections - I work abroad in a creche and had a work ban whilst pregnant until I had been tested as I was obviously dealing with under 3s all the time - nappies, runny noses, you name it and as a shedding virus was exposed.
As it can cause cerebral palsy, deafness or eyesight problems it really should have been taken seriously by your SIL.
My blood test showed I had had it previously - just to warn you though my lab results could not say how long before and I was at the end of my second trimester so although it was a relief to know I had antibodies it did not and could not reassure me that I hadn't been infected for the first time in the first trimester iyswim.
But my DC was fine. I also discovered that despite being a cat-owner in the past I had never been exposed to toxoplasmosis - another torch disease.
This is the other side of it of course - you might be told you have no immunity to cmv and have never had it. Great. But then you realise that like me with toxo there is not an awful lot you can do ( besides hand washing and avoiding known SIL in your case): I can avoid handling raw meat/cats etc but ultimately you hope you are low risk.
I have not been retested for toxo this time around - it is something they recommend you do throughout if you are pregnant with no immunity as there are drugs they can give you if you get it while pregnant but to be tested and retested continuously over 7 months would be stressful and there are false positives so decided against.
This is also true of CMV -you usually need a blood test two weeks after the first one and there are antiviral drugs available eg ganciclovir if you catch cmv for the first time while pregnant but the latter is only used if benefits outweigh risk as it is unknown whether the drug itself can cause birth defects.
Your SIL has been astoundingly selfish or naive or plain ignorant.
As to travelling to Ireland only you can make that call but they need to be aware you might not be up to it full stop.
Hugs and good wishes being sent your way x