look at the starting out series from everyman chess they explain the concepts and positional/tactical ideas of the openings
Opening book with good explenation
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Chessbase12 reader is free to download and contains a database. Since technology has advanced we can use it for study purposes. See how to download HERE some free material and see some more suggestions in the following numbered links.
If you prefer books you can find discounts here and here, but who can suggest only one book? That's because you know better what you need.
Is Irish supposed to be favorable?