The Irish teenager has agreed to join Leeds from Limerick.
Leeds United lost 2-0 to Liverpool in the EFL Cup on Tuesday night.
Goals from Lee Bowyer (2), Mark Viduka, Eirik Bakke, Dominic Matteo and Darren Huckerby sealed an impressive win for Leeds United.
Blackwell was dismissed after just over two years in charge.
The Irishman was a one-club man at Elland Road.
At 20 in the countdown is a defender who remained a stalwart of the side for over a decade, making over 500 appearances in the process.