What to pack for Ireland and Scotland in September

I was in Edinburgh and Glasgow in the second half of September several years ago. I took layers: wool blend sweaters, a knitted wool vest, long sleeve cotton T shirt /turtlenecks, an autumn weight quilted jacket, a windproof hooded jacket (though not waterproof, it was okay only for light rain for a short time), also a lightweight hat that was worn, of course,several pants. Edinburgh is quite cloudy and windy. Don't bother with an umbrella- likely to be useless in minutes b/c of the strong winds.

We were in Scotland about that time 3 years ago. Bring clothing that can be layered then your ready for anything- short and long sleeve tops, lightweight merino wool sweaters, waterproof raincoat/jacket.

I also packed lightweight cuddle duds for a base layer and a down vest. I never needed either.

However, we were traveling with another couple. She was bundled up almost every day with heavy jacket, gloves and cap. When looking at our photos, you'd think we were on different trips.

Layers, including a waterproof, hooded rain jacket that fits over the other layers. My go-to layers are a puffer jacket (for September, the one without the hood) and a rain jacket. Both hip-length. I don't bother with waterproof pants because I'm generally not hiking. It sounds like you'll most be in the cities, where they have handy shops, pubs and cafes you can duck into if the rain turns to a downpour. It should pass quickly. As mentioned, an umbrella is useless. A tip I read on the Forum is to wear a ball cap to prop up the bill of your jacket's hood. Ahead of my February trip to the UK, I was able to find an insulated one. Never used it.

Hello Kimberly from another Kimberly!
I usually pack in layers no matter the season for Ireland (and Scotland.) A rain coat is super useful even if it doesn't rain as it also doubles as a wind breaker. Typically I have a fleece coat to layer under and a mix of long sleeved/short sleeved shirts. Comfortable shoes are important, too, whatever that looks like for you (I bring my birkenstocks, running shoes, and a pair of nice shoes.) Living in the PNW, the Irish weather is normal to me so I don't usually pack any rain pants or water proof shoes. And, as others have said, an umbrella will not be of use even if it is raining. You might also toss in a pair of shorts or a warm weather item (or two) just in case. This has been a warm summer in Ireland (was 80's when we were there in July) and it was unseasonably nice the end of April/beginning of May (mid-60's and sunny.) I'd also suggest throwing in a scarf to help with wind and cool temps. The only predictability of Irish weather is it's unpredictable. And if you find the temperatures too cool, they make lovely sweaters in Ireland. ;-)
Enjoy your trip!