The Lord and the Banshee by Gillian St. Kevern

Death is not the worst thing that can happen.
Thomas Cross, Lord of Foxwood, has received a double blow. The discovery of his terminal illness is followed immediately by the news that his longterm partner Pip is marked for death by the banshee of Connaught Castle. There is no cure for Cross’s condition, but there may be a way to save Pip—at the cost of his remaining time.
Recruiting his adopted son Julian to aid him, Cross travels to Ireland to free Pip from the banshee. As each successive encounter with the banshee leaves him closer to death, Cross grows more dependent on a dangerous fairy relic. But the gifts of the fair folk come at a price. What is the cost of Pip’s safety?