Lake District, UK best home base

In October you are correct...it will be far less crowded in the Lake District and the weather should be quite nice. You'll definitely have a mix of sun and rain but that's all part of the experience there.
There is easy walking from all three places you mention. Since you won't have a car, the bus system in the Lakes is very easy and you can get just about wherever you wish to go.
Keswick is where I normally base myself when I go (which is every couple of years). It has the best hub for the bus system and has direct routes to many places. It also has the advantage of a boat launch that circles the lake with several places you can get on and off to walk or stop off and walk. The trail around Derwentwater is relatively flat and you can get off, walk to the next dock, and then get back on. There are 8 docks around the lake. You said that Keswick isn't right on the lake...it actually is. From the main square it's a 5 minute walk to the shore. Beautiful views from the shore and a theatre where you can watch wonderful performances in the evening. Keswick is the most "real" of the three towns you are looking to stay in. Has a nice sense of community with local pubs and shops.
Grasmere is smaller than Keswick. It is extremely picturesque but very touristy. Good walks from there around Rydal Water and the surrounding hills. Wordsworth lived here and you can visit his home, Dove Cottage, and visit the small church where he attended and is buried. Rydal Mount, his larger home, is also nearby.
Ambleside is a busy little place - quite touristy - mostly streets of B&Bs with restaurants and shops. Pretty but not quite as picturesque as the other two.
Coming from Ireland will you fly? If so, Manchester is a good place to fly into. You can take the train to Oxenholme and then transfer to the smaller train that goes to Windermere. From there you can take the 555 bus straight north from the station. It stops at all three towns so no matter where you decide to stay you can get off there.
You can definitely hire a driver if you like but it will be quite expensive.
My favorite places to walk in the area, besides around Keswick and Rydal Water, are Buttermere, Castlerigg (go to the stone circle there - there is a bus stop), St. John's in the Vale, Borrowdale, Hawkshead, Coniston, and Sawrey. Those areas all offer fairly flat walking with beautiful countryside and villages.
As for your vegetarian, pescatarian, no dairy needs, this shouldn't be a problem. Restaurants in this area, and England in general, are very attuned to these kinds of dietary needs. Even pubs now have a great selection of vegetarian food and diets with various restrictions.

Hi Stephanie,

We spend quite a bit of time on Vashon and might have some mutual friends. But I am writing to suggest yountake a look at the programs offered by HF Holidays at their country house at Derwent Bank near Keswick. The house is right on the lake (Derwentwater) with a lovely garden and lawn sloping down to the water. We had a lakefront room that was very nice.

HF Holidays is a cooperative that owns country houses all over the U.K., offering 3- and 4- night all-inclusive holidays with a focus on walking but also on focused interests such as photography, cultural pursuits, yoga, historical sites, and other interests. Take a look and see if one might fit your time frame:

https://www.hfholidays.co.uk/country-houses/derwentwater/?&page=2&page=3&page=2&page=1&page=2&page=1

There is a "Focus on Light" weekend in October but too early for you. But your photographer friend will find opportunities for good photos all around, should you choose one of the regular walking Holidays. They offer walks at various levels, with transport provided, Nd it is a great way to see the Lake District. We really enjoyed our stay there in 2016 and are going to an HF Holiday home in Dorset next, in May.

The rooms are very nice, with ensuite bath and all comforts. Meals are excellent, and they can provide for vegetarian or pescevegetarian diets. You could ask about the dairy-free and I suspect they are able to accommodate that as well. The group camaraderie is a highlight.

For this location, you could travel by train to Penrith from either Glasgow or Manchester. They have a shuttle that will pick you up at the Penrith Statuon in late afternoon and take you directly to the house. You need to reserve and the cost is around £10. Once there, you can rely on their transport to the trailheads for walks, or go on your own by walking the short distance into Keswick and taking local bus or boat from there.

Thank you for great feedback.