After some internet browsing I think I have come across some suitable riders for the Irish army, Griping Beast's Scots-Irish Mounted Ceithern. Of course it would be stupid to order just one pack of miniatures... ;-)
Gripping Beast Mounted Ceithern
Looking to maximise the postage I have also decided to order a couple of Romano-British cavalry to fill the gaps and a box of the new plastic Dark Age Warriors.
Gripping Beast Dark Age Warriors
I don't really need the Dark Age Warriors but as I already have the Gripping Beast plastic Vikings and Saxons insulating the loft I am considering using them to raise Late Saxon and Norse Sea Raider armies for Dux Bellorum. Each box should provide pretty much all that is needed, but the unarmoured generic Dark Age Warriors will fill any requirements for skirmishers, bow, unarmoured spear etc.