I think it was a mix, I think the players we have available now are better but, as the years following 2015 showed, we definitely had the players then. What England lacked was the cutting edge and the ability to grind out the wins in the tight games, which they gained when Jones first took over. Lancaster also must carry a lot of the criticism as he bottled it big time. England had been developing a set style of play which had been successful but not quite the finished article, always coming 2nd in the 6N. When the world cup came, that all got thrown out the window and we played a game that was not one we had used for the past 4 years. Tellingly Farrell at 10 in particular was a big mistake (our record with Farrell at 10 is much worse than Ford at 10).
Lancaster is gone for good, he will not want to be involved in the England setup ever again. I think he is very skilled but he doesn't seem to have that stubborn self belief to really be a head coach. Farrell did have that but it was to the detriment of the team as it seems defensive considerations were overriding everything else.
Also you have to match a coaching style with a team philosophy and players. It may simply be that the English coaches of the 2015 team were better suited to the Irish mentality than an English one. Ultimately we will have to wait and see how Ireland do in the next world cup. For all you know, Farrell and Catt could bottle it again and Ireland could crash out in the group stages...