As somebody else mentioned, there is a common travel area between the UK and Ireland and each country's immigrations officers are meant to enforce each other's immigration laws too. It is therefore the immigration officer's job to check your credentials at LHR even if your destination is Ireland.
Return flight details must rank among the top 3 things that an immigration officer will want to see as it usually is a condition for being allowed into the country. It therefore makes sense to have a printout of that easily accessible when approaching immigration booths.
Of course, none of that excused rudeness, however.