Hi Abigail, you mention a separator toilet - is that a dry toilet with no flush water at all? Such as the Seperatt unit? If so, you're correct and we're dealing only with grey water. If you have any flush water (for example after an Aquatron faecal separator unit) then you'd need to factor this into your overall treatment system design.
Vis a vis the grey water - we generally get enough rain water here that I'm not really in favour of expending a lot of cash, energy and resources on getting grey water clean enough to use in a polytunnel. Certainly in very hot dry weather like we've had this summer it's worth it, but not most of the time. We're using a basin in the kitchen sink at present for watering pots; and a bucket in the shower for flushing the indoor toilet. Both of these are very low tech, and save a lot of water.
For filtering the grey water, I'd go with the woodland filter option you mentioned. You may wish to buy a grey water filter in a local hardware shop for the initial screening of the grey water, or you can filter the grey water through a small constructed wetland instead. If you wanted to use a gravel reed bed, you'd still need the grey water filter to remove solids first.
If you already have trees, there's no need to plant willows. The existing trees will take up a lot of moisture and nutrients. If you want to get a good distribution of grey water (i.e. if your soil is heavy, like much of Roscommon) then a splitter unit may be helpful. These can be ordered from ribbit.ie in Co. Sligo. It's a simple splitter unit that works really well. Technically you should follow the EPA Code of Practice (free to download if you follow wetlandsystems.ie/links.html) unless you are simply using the system to irrigate the trees to enhance their growth and development, in which case planning permission may not be necessary.
Call or email if you have any queries as you go forward.