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This is probably an easy one, but I can't think of the right combination of keywords. I read it as a hardback around the late 1980s to the early 1990s. It involved a brother and sister who somehow wound up back in early ages of Celtic Ireland. They meet a young man who turns out to be Cuchulain, although I want to say that he used a different name, so it took the children a while to catch on. It was definitely a fantastic take on it, as I remember Cuchulain using Gáe Bulg, his magic spear. Specifically, there was something involving chasms belching poisonous gasses, but he could breath with his mouth by the spear (which does not seem to be among the legendary qualities of that weapon).
I vaguely remember that the children first encounter Cuchulain by a body of water, I think as he was bathing. When the children are returned home, they return with a carved comb of bone or ivory. I remember they also came home with a case of lice, which required boiling the comb. Things that stick with you... There might have been a sequel, but I don't remember any details if so.
I think (and this is very tenuous) that the cover had the brother and sister stepping out of a forest, hand in hand, with Cuchulain down by the water's edge, as per that first meeting.