It depends if you'll pick him up in the draft, or if you want to take him from the market.
right now, arrive to be a U21 ireland national player is not extremely difficult.
You need the patience of train your player in single position for 2 years , and it's realistic that a very good trained 20 y.o. player will be part of the U21 team.
in the market you can buy at 350-400K € a good player.
he must have good potential and a good sum of skills.
soon there will be a good 19 y.o. player on the market ... stay tuned .. :D