I stayed in Germany for three months, summer of 2018 but my Schengen visa was soon to expire and I needed to exit for a minimum of three months. I went to the UK and was stamped in for 6 months. Then I went to Ireland and was stamped in for 3 months. When I left Ireland and returned to England none of us on the plane went through any passport control leaving Ireland nor entering England. I asked a fellow traveller who lives there and she said it was abnormal. I asked an airport employee and he laughed and said "We let anybody in these days." I stayed for Christmas and returned to Germany in the Schengen zone after being absent the required three months. Again when I departed England there was no one inspecting or stamping passports and when I arrived in Germany, in a small airport, I was warmly greeted and welcomed by the customs officer but he also did not stamp my passport. Is this usual? Will I run into any problems down the road? Do I need a stamp?
If your situation warrants stamping your travel documents, then you are indeed supposed to get a stamp every time you enter or leave the Schengen area. The UK and Ireland operate differently and Schengen border guards should not rely on this to enforce Schengen rules.
If you do not have a stamp and are challenged, it's up to you to prove you didn't overstay in the Schengen area. You can rely on any material evidence of your travels to establish that (receipt and credit card charges, stamps from other countries, airline boarding cards).
I think you should have gotten a Schengen entry stamp, and the absence could bring minor problems later if you are there on a short-stay C visa. Do you have a Schengen exit stamp from your first visit?
Keep documents to prove that you were outside the Schengen area. Carry them with you the next time you leave, and make sure that you get an exit stamp.