Biking through Ireland in September?

I think that planning for rain is something you always have to do for a
biking trip, and in Ireland, especially so. But if you're used to biking in
the rain, it's not a big deal. Unlikely to get major storms, just drizzle,
rain, etc.

On one trip, we had 9 straight days of sunshine and mid-70's in July,
that was a complete statistical anomaly...

Thank you!

How wet is too wet? Do you have adequate clothing … GoreTex jacket (with a hood), and rain pants, shoe covers? We did a ride from Galway, then south in June 2011. Two mornings were rainy, and we were dressed for the weather. Another day, a sudden downpour arrived out of nowhere, and we didn’t have time to put on rain gear before we were soaked. We spent an unplanned night in a B&B in the closest town, drying out. The rest of the two weeks was dry.

In mid-September 2018, supposedly a rainier month than June, going from Galway then north, there were again only 3 days with rain, and then just part of each day. How far you plan to ride each day, and whether you absolutely have to make it to a prearranged destination each day could affect your trip. Having flexibility on timing and itinerary could help you deal with rain situations. Some sights and attractions had shorter hours, or were not open, compared to the high summer season.

Not in Ireland, but still September, my husband went on a 10-day organized ride in Italy in 2022. They had mostly rainy days, but were dressed for it. Rain didn’t prevent that trip, but he had clothing strung throughout the bathroom to dry out each night.

In Ireland, being able to take a bus (coach) to the next destination, or pulling in to a town for the night if the weather is too soggy could help.

Something else to consider besides the weather will be road conditions in general. Most of the rural roads are narrow with no shoulders to speak of. We're not cyclists ourselves but I recall encountering some on our previous driving trips and marveling at the courage of those who chose to engage in the activity - it's not the most bike-friendly country we've visited.
You might find it helpful to preview some of your proposed routes by using the Street View feature on Google maps to check out road conditions ahead of time.

Thanks so much.

Although you might not want to wait, the Irish government just announced a 1.4 billion euro investment in bike paths to be complete by 2040. First completion to be Dublin to the Shannon river. Of course there are already some bike paths in place, as in around Westport.