That's an interesting point, re Irish youngsters.
Pre and during EU membership, Irish people did and do have the same rights as British people. By that I mean they can join the Army, Police, become an MP and even become PM as an Irish citizen.
It'll be interesting to see if that changes post EU membership.
EDIT: Found this
www.freemovement.org.uk
The UK government’s policy is that Brexit will not affect Irish nationals at all. Other EU citizens have to apply for a new “
settled status” or risk losing their right to live and work in the UK after June 2021. But the government’s
position is that Irish people, whether existing residents or future migrants, have a general right to live in the United Kingdom which is entirely separate to their rights as EU citizens:
Irish citizens enjoy a right of residence in the UK that is not reliant on the UK’s membership of the EU. This means that Irish citizens do not need to apply for status under the scheme. Nonetheless, Irish citizens can make an application under the scheme, should they wish to do so.
So it appears that it won't affect Irish players, young or not, from moving to a club in Britain.
2nd EDIT:
Also found this,
Premier League clubs to be limited to three under-21 signings per transfer window with all foreign players subjected to a point-based threshold to be eligible to play in the UK
www.independent.co.uk
The new regulations, which have been approved by the Premier League and English Football League, will also see Premier League clubs limited to
signing no more than three players under the age of 21 in any single transfer window, meaning no more than six per season, in an effort to help produce homegrown talent in the professional leagues.
A joint-statement issued by the FA, Premier League and EFL confirmed that the regulations have been approved by the
Home Office, and that they will come into effect from 1 January in time for the next transfer window, with
Fifa’s current rules meaning that the end to free movement from the EU to the United Kingdom means that signing players under the age of 18 can no longer be permitted.
So it now appears that it's the FIFA rules that prevent clubs from signing players under 18 from Ireland rather than British immigration rules/laws
Either way clubs won't be able to sign any players that are under 18 from any country.