I think you have to look at this slightly differently
In general, if a direct flight isn't available, the average tourist won't come.
In the case of Dublin, that's reducing long-haul tourists in particular.
In the case of Cork and Shannon airports, it's reducing European tourists.
In the case of the smaller airports, it's reducing tourists from the UK.
And that's where a change in charges can come in.
With the margins that airlines operate on, even a saving of E3 per passenger can seriously affect the viability of a route. They can still obtain the same yield from the passengers, so effectively, it's like starting a flight with an extra E200-E600 to cover costs. You'd be surprised how many extra routes become viable from that.
And with the extra routes come extra tourists.
Now, I've written before on other threads about how Ireland seriously needs to update its tourism offering.
In particular, we're stuck in the 1980s where tourists got bussed around the country or brought/hired a car. With the rise in independent tourism over the last 10+ years, the coaching option has become almost obsolete.
But you want to travel round by public transport?
Look at the tourist areas...
Schull to Killarney - Please take a massive detour via Cork
Cllifden to Westport - Galway looks forward to welcoming you
With a reduction in travel tax and airport costs, and without a more comprehensive review of tourist needs, there's likely to be an increase in short-stays in Dublin and to a lesser extent Cork, Limerick and maybe Galway.
And that's about it.
Of course, that's completely outside the realms of an aviation thread, but yet without it, where do the airlines get their passengers.