With a population of just 4.7 million the Republic of Ireland sold an impressive 10.9 million books last year to the value of €130.9 million. That of course is according to Nielsen BookScan, so by no means the entire book market.

The Bookseller this past week reported,

HarperCollins UK is doubling the size of its team in Ireland through a number of new roles and appointments as part of the company’s “commitment to the thriving Irish book market”.

HarperCollins UK’s CEO Charlie Redmayne spoke of the company’s

strategy to build local publishing in territories around the world,

adding,

Ireland is a hugely important book market for HarperCollins and this move will build on the great work done by our existing team over many years.

Of the €130.8m in sales last year just 24.3% (€31.8m) were Irish-published, which of course means over 75% were imports, safe to say almost all from the UK and US.
And that will be a key reason why HarperCollins rates the Ireland market so highly.
Across Europe Norway, Slovakia, Denmark, Croatia and Finland are all comparable in population size, but each requires expensive translations, both for imports and exports.
Ireland on the other hand, as an English-language market, can receive imports from and make exports to the USA, UK and Commonwealth countries with almost no changes bar perhaps some co0ncessions to British/American English spellings.
No surprise then that HarperCollins sees Ireland as a market worth investing in.