Firstly, I'm Irish.
Secondly, beating what is put in front of you is easier when the teams that are put in front of you are poor (obviously).
Thirdly, Belgium are a nothing side outside of KdB and quite possibly the single most overrated side in world football over the last decade (club or country).
Fourthly, the German side that put England out previously was a brilliant team. Claiming that they were an U21 team is disingenuous at best. A team containing Neuer, Lahm, Boateng, Khedira, Schweini, Özil, Klose, and Muller is a hell of a lot better than anything Southgate has faced let alone defeated in his English managerial career.
Other than Iceland, England have generally been very good at beating sides that they are better than and getting knocked out by sides that are equal to or better than them. If I was being generous I'd say that Southgate has merely continued that trend but I don't think that England should have been losing to Belgium at least.