Looking at the position of your proposed border I wondered about the advisability of using hydrangeas so near to your house. They would be pretty while immature but some grow five or six feet high and as much across and that's all you would be able to see from your windows after a few years.
If you want them you might consider siting them in a bed further from the house where they would not obstruct your view, and plant something lower along by the path - just a thought?

An unrelated suggestion, me just being an interfering old biddy, but had you thought of planting some sort of shelter belt in the lawn to give some protection from the strong SW winds? It would be quite challenging to grow taller plants without.

Regarding your planting ideas, there's a couple of things to consider - Hydrangeas are deciduous, so although you're imagining all those different flowers in summer, you'll be looking at a bunch of dead sticks all winter, for at least six months of the year. Agapanthus disappears below ground in winter, as does Crocosmia, so all in all, there'll be nothing much to look at for six months.

Hydrangeas also get tall and wide over time, and its not possible to keep pruning back H. macrophylla varieties to keep them in bounds - only having agapanthus and crocosmia means they'd be rather lost against all the hydrangeas, plus agapanthus and crocosmia will do much better somewhere with more sunlight than what's available there.

You can certainly have Hydrangeas - but you'd need to mix in some evergreen shrubs so that it doesn't look awful in winter. The idea of a shelter belt is a good one to cut the wind down, but you're still restricted to some extent by the limited amount of sunlight - the conditions you describe are technically shady. For lower growing, evergreen, shade tolerant shrubs, consider Sarcococca varieties, Prunus 'Otto Luyken' , Box and Skimmia. Liriope 'monroe white' is very tolerant of a lot of shade, flowers in autumn.

Thank you both, lots food for thought there. Steragram re the wind barrier idea, it's been domestically a very contentious issue as my partner jealously guards the rural valley views( a legacy of many years of urban life) and it's all about compromise, the old local boys well accustomed to the weather here stop off and advise the value of a "sacrificial barrier" but the negotiations go on. Did you have something in particular in mind?

You would be surprised by how wide a strip is sheltered per metre of height of even a shrub. You could have shelter for a reasonably wide bed without losing any view. Test it by putting canes in different places in such a position that you can see over the top of them. Then if you really want hydrangeas perhaps have them further away from the house to one side, remembering that they will eventually get quite tall. I would strongly advise you to take notice of the locals. Ask them where they would put a shelter belt and what they would use, bearing the view in mind. It could save you a lot of time money and disappointment.
Next thing is, are you intending to spend a lot of time on the garden or do you want it to be labour saving? Very different planting approaches!

Some gardens have used gaps in a taller shelter belt to "frame" a distant view and it can look very attractive and inviting.

Do you actually live in Cork or the English midlands?

For a north facing evergreen labour saving border near the house you might consider some low growing variegated evergreens such Euonymus fortunii . There are several varieties, bright and cheerful and needing only an annual trim if you think they need it.

Thank you Steragram, will have a go at it.