National team players to to play there.
Teams get supported.
Due to success of the English teams relatively to Irish and the presence of Irish international players in the UK game there is interest leading to newspaper and radio coverage. When tv becomes more prominent it has two impacts houses that have extra channels, all UK, would have English soccer on offer, also due to lack of tv at Irish matches it means that tv viewers in the pub or at home only have one option available to them.
Eventually interest due to the population reaching enough exposure to it becomes self perpetuating, people get their favourite teams for various reasons but if it isn't your dad or brother having supported them it could be an Irish presence at the club or any other of random reasons. If the number of people passing down interest is high enough people will follow it and most of them will have a favourite team for some reason or other while a smaller number will be interested in the league in general. It's a long running soap opera and supporting a a team adds to the drama.