I used Padraig Hughes Windows from Castlebar in the end, and I would be very happy to recommend him.
He gave good advice in terms of what design/openings/orientation to go for.
They use Carey Glass for the units, with argon fill, and E-glass.
They make up the frames themselves in their own workshop.
They did a great job on removing the old windows and managed to not break any tiles in the 3 tiled rooms, couple of slight bits of damage with an angle grinder where they had to cut out brackets, but nothing major.
They got the old ones out and the new ones in (11 windows) in one day by using 3 guys. Then they spent 3 man days on making good. They were as neat and tidy as I would expect.
The new windows were a lovely fit, and they did a fantastic job in making good the plaster on the inside and out, doing a proper job with bonding / skim on the inside and cement plaster on the outside.
Good communication, very good product and very well fitted and finished - I am very happy to recommend them.