I write as someone who went through the Irish Junior Cert, Irish Leaving Cert and Oxford, although that was Oxford as a postgrad not an undergrad.
First, 12 As in the Junior Cert is not really the equivalent of 12 As at GCSE. Okay, maybe it is the equivalent of 12 As, but not 12 A*s, IYSWIM. A smart student with good teachers who works relatively hard should be able to get those grades. The Junior Cert is not the same standard as GCSE - unsurprisingly, as it is sat by 15 year olds after three years of secondary school whereas GSCEs are sat by 16 year olds after five years of secondary school.
Secondly, why you are jumping from Ireland to Charterhouse rather than looking at a fee-paying Irish secondary school, given that the fees are a lot lower in Irish fee-paying schools and the curriculum would be more familiar? I think it would be a real struggle doing A-levels from Junior Cert standard. Although I do note that you are envisaging your DD as doing one year of Leaving Cert studies before moving across, do not expect a smooth transition. At this stage it's worth thinking about what four subjects your DD would be planning to do for A-level and comparing where she is with the GSCE standard, e.g. by looking at GSCE past papers, marking schemes, etc.
Thirdly, you mention that the school your DD is at won't get her into the university she wants to go to. Is that Trinity College Dublin, or is it Oxbridge or is it Harvard/Yale/Columbia? If you look at undergrad admissions to Oxbridge from Ireland it is generally single digit, e.g. Oxford 2013 had 97 Irish domiciled applicants of whom 9 were admitted. That's much lower than the 23% acceptance rate for UK applicants, especially given that the small number of Irish applicants will inevitably be the top rank academically. Do I think Oxbridge discriminates against non-UK applicants - yes, certainly, otherwise it would be flooded with them. Will your DD have a better chance applying to Oxbridge from a UK independent school like Charterhouse than from an Irish school - almost certainly, but will her chance still be that high, I doubt it. I can't speak in relation to the top tier American universities, but I think further down there will be plenty of American universities that might be generous to your daugher, esp because of swimming and music and because of Irish connections. Oxbridge is not a very friendly place for Irish people, to be very frank, and the only girls sport that counts for much is rowing. For colleges singing ability (for choir) is more important than piano aptitude.
It is perfectly possible to do a degree in Ireland, do well, and then go on to do a postgrad in Oxbridge or the U.S. or anywhere else in the world. It's a well-trodden route. I didn't even have a first in my undergrad and I got into Oxford. When parents have a really bright child they can sometimes feel that they have to do everything they can to support their academic dreams. However, I know from experience close to home that this is not always the best thing for their child. I don't know your DD so I have no idea how she is placed, but I suggest that you think carefully about any decisions.