I am not a big fan of First Day Covers. I can certainly appreciate how the accidental discovery of a stamp posted on its first day of issue would have added value. Genuine Postal History.
Certainly Irish FDCs from the first commemorative issue thru the 1930s are highly desirable. After this prices become sensible and by the 1970s FDCs have flooded the market. Postal History is now pre-packaged.
Most market stalls and Collector Fairs have FDCs at very low prices.
Effectively FDCs have ceased to be Postal History and become Philatelic Souvenirs.
The 1954 Marian Year issue is genuinely posted. It is not an illustrated cover. The 1969 International Labour Organisation issue has also passed thru the regular mail but this is an illustrated envelope.
The 2013 President John F Kennedy issue has not passed thru the post. It is an illustrated envelope and has a special pictorial cancellation. I bought this cover in Dublin. The Philatelic Shop (of An Post) sells FDCs for one month after a stamp has been issued.