Some of this has been answered by Northernyellow, but i will give you my take on it (and probably bore you all)
Over the last 6-7 years there has been a real drive to improve the standard of young players, Michael O'Neill was partially behind this move, he was very concerned at the high percentage of NI kids who were coming back home from England/Scotland after leaving at 16 and being released at 18/19, alot of these kids ended up quitting the game, so the IFA started looking into the reasons for this and found that the main reasons were the culture shock of the full-time environment and homesickness, it was noted that quite a few of the young players were also picking up injuries a few months into playing fulltime football due to their bodies not being used to the intensity of full time training which was hurting their development and was then leading to homesickness. To try and counter this the IFA in conjunction with Michael O'Neill and Jim Magilton setup a development programme called ClubNI were the most talented kids (nomally 25 in each year group) are given training two nights a week by FIFA Pro license holders like Jim or others this is alongside the two nights a week they train with their own clubs, the scheme has been constantly evolving over the years, this year a full time residential academy has been setup for the 16 year olds to better prepare them for living away from home and training every day.
The ClubNI project has no doubt improved the level of player coming out of NI, whilst the number of players leaving NI for England/Scotland hasn't dramatically increased the level of club they are going to has, we have players in the last few years signing for Liverpool, Man Utd, Chelsea, Wolves, Leeds, Aston Villa, West Ham, Southampton, Rangers, Celtic etc.
As well as the improved development of young players, the appointment of Ian Baraclough as our u21 manager had a positive impact, he prefered to pick u21 players who were playing regularly in the NIPL (NI Premier League) or LOI (League of Ireland) than kids playing non-competitive u23 football in England/Scotland and the team had its best run of results in its history, this increased interest among scouts in our local NIPL. Out of Baraclough's squad; Paul Smyth moved to QPR, Brad Lyons to Blackburn, Bobby Burns to Hearts, Gavin Whyte and Mark Sykes and (possibly Joel Cooper) to yourselves and these are all lads who were deemed not good enough at 16-18 to move into full time football.
The last contributing factor is the NIPL, the standard of the league has improved over this last few years. Three years ago the owner of Purple Bricks bought Larne FC, a club who were in the second tier and going nowhere fast; their ground was a dump, their fanbase was virtually non-existant and he set about investing big money in buying their stadium, building new facilities (bars, suites, changing rooms etc), a new stand and (two more in the pipeline) and really engaging with the local community but more importantly he turned the club full-time, this was the first club to go full time in NI, in his second season they were promoted to the top tier, this show of ambition drove Crusaders last season to go effectively full-time, a new millionaire investor came into Glentoran last season and they are now mostly a full time team, the biggest club in NI; Linfield has now also made the decision to go full time when the new season starts, these clubs are also giving young players opportunities in what is a competitive league, this past season both Linfield and Glentoran have given debuts to 15 year olds (both have now signed for English clubs).
As well as the clubs i have mentioned, some of the other clubs who don't have the finances to compete with the bigger clubs have turned to promoting young players instead, by doing this the young lads who don't go over the water at 16-17 years of age are getting great experience playing in a highly competitive mens league, if they have the ability they are then getting picked for the NI u19's or u21's and getting a chance to showcase their talents at international level, the fees involved in buying players from NI are also quite low in English terms, most of these lads will be sold for £100-200k but they arrive in England/Scotland at maybe 19, 20, 21 years of age with 100-150 games under their belt and international experience.