I would try and keep it really simple to avoid future issues and complications.
I am not sure if I had a disabled child that being left an apartment in trust in another country would be really beneficial to me. Could I rent it out? But I would need to pay tax and open foreign bank accounts and have an agent etc. Could I use it for holidays? But maybe I would like to go somewhere warm and not go to Ireland. Could I afford the upkeep? Maybe I would be entitled to services for my child but now that I have this apartment as an asset I am deemed by the uk government as too wealthy and they say I should pay for the service myself.
There is nothing wrong in deciding to give the bulk of your estate to one person, if you think they have the greatest need. I would be in favour of directing the executor to selling the house and then dividing your estate by %. I want to give 2 % to Mary, 2 % to John, 2% to Peter, and 94% to Michelle. I would separately write a letter of wishes to Michelle, saying - if any of the inheritance is left when you die, can you please divide it equally between Mary, John & Peter.
There is inheritance tax to consider as well, I would think a niece could only inherit €33K (or thereabouts) before they need to pay tax. So if you leave her an apartment which she cannot sell, the revenue would still put a monetary value on the inheritance and send her a bill for the tax. So maybe leaving everyone a small % means that less is paid to revenue and more people get a slice.
Ultimately, talk to your solicitor, get them to explain the implications of what you want to do. Tax, potential pitfalls, other solutions. But say your niece is 45 and she lives until she is 95, do you really want someone in 2071 dealing with what you decided in 2021, the world will be a very different place then.
I am not speaking form direct experience but my husbands niece (in UK) was left a tiny cottage in Ireland in late 2019. So far my husband has cleared out the cottage, maintained the garden, paid the esb and insurance and kept an eye on it since (at the request of the solicitor). The niece discussed visiting it once last summer but we not aware if the visit actually too place. So far she has made no effort to take ownership, sell, help progress probate, nothing really. It appears she does not want to engage with the process. It probably was not the wish of the dead owner that things happened this way but here we still are in a limbo.
So my advice is keep it simple, direct the executor to sell and distribute the estate in cash to the beneficiaries. And let the future look after itself.