As per 80s book similar to Narnia but with a more Celt feel, allusions to Finn MacCool and eating "the salmon of wisdom", it is The Wizard Children of Finn, by Mary Tannen with the sequel being The Lost Legend of Finn. I incorrectly remembered the hero, who was Finn McCool with his spear, Birgha.
Fiona McCool, a wise-cracking eleven-year-old, and her dreamy thumb-sucking brother, Bran, are lured into the woods by a mysterious whistle. The whilstler turns out to be Finn, an enchanted boy who has been watching the McCools, hoping to make friends with them. Fiona and Bran are caught up with him in a magical spell that flings them back through time to Finn's homeland, the Ireland of two thousand years ago. They journey on foot across ancient Ireland -- over two rivers, battling wild beasts, traveling toward the awesome celebration of Samhain, where their fate is to be decided. Fiona becomes Finn's poet, spinning their adventures into poems, while Bran remembers the tales perfectly and recites them. Together, the McCools document the story of Finn, a boy on the brink of manhood whom they grow to love, but cannot begin to understand.
This review does a good job of summarizing The Lost Legend of Finn:
The sequel to Mary Tannen's The Wizard Children of Finn, her The Lost Legend of Finn, has Fiona and Bran McCool again traveling back in time to ancient Ireland, this time though very much deliberately, to hopefully find information and answers regarding their mysterious father. However, Bran's magic goes a bit awry, and instead of traveling back to the ancient Ireland of 2000 years ago (which they had wanted to do and where their friend Finn is leader of the Fianna), they end up in early Mediaeval Ireland, a Christian Ireland being threatened by Viking invaders from the still pagan north. An ancient (and perhaps timeless and everlasting) druid named Biddy Gwynn transforms Bran and Fiona into ravens, sending them into a lost chapter of the Legend of Finn. And within said story, within the legend, Fiona and Bran again encounter their friend Finn (but only as raven observers, and thus he is unaware of their presence). However, much to their consternation and surprise, they also encounter their own mother (Sadie), as Sabdh, one of the Everlasting Ones, in the form of a red deer. Sabdh, who is being pursued by Fear Doriche, the Dark Druid of the Men of Dea, places herself under Finn's protection (as under his protection, she no longer has to be a deer), actually marrying him, and Bran and Fiona now realise in astonished consternation that their erstwhile companion Finn (from their previous adventures in ancient Ireland) is much more closely connected to them than they ever could have guessed and imagined, that he is in fact their father (something that I already kind of expected when I was reading The Wizard Children of Finn, but it sure does feel kind of nice to have been indeed proven correct with regard to this, to have actually guess correctly).