I know you probably feel it's never-ending supporting charities and sponsoring family, but kudos to your niece for putting in the work, and it's very likely that her own parents have also sponsored her generously. As any amount is gratefully received anyway it can be as little or as much as you want it to be - you're in charge of that. You could always say that as you already donate on an ongoing basis you're just putting in a token amount - think that is fair enough, and nobody's going to think any less of you.
The Republic of Ireland and their tensions with migrants.