expect it to be very variable - cool and rainy or bright and sunny - you can look at historic averages but the weather is very changeable.

It almost certainly won't snow, but almost anything else could happen.

Agree with all. We had a lovely bus driver for our trek to Kilkenney from Dublin and he referred to the weather then in March as "bi-polar." We went from sun to SNOW to rain to sun within 6 hours. 55F to 32F to 40F.

In June, that translates to "sun to pouring rain to soft rain to sun" in 6 hours.

Layers, my dear, always layers.
However, one can cheat the system if willing to look darn ugly. Those plastic shower caps available on board ship and simple dollar rain ponchos can make you weather ready pretty darn fast.
AZ

As AlessandraZoe said.... layers, layers, layers. We went in July and had, for the most part, really lovely weather. But also experienced a lot of mist, a torrential storm on the west coast, and lots and lots of wind. We kept light, compact northface windbreaker jackets and umbrellas in our daypacks just in case.