Note: Other than a detachable head, there isn't much in common with the standard Irish myth, and this makes use of a magic system. I'm also fine with a little hand waving if I must, I would just rather look at other options first.
In this setting, the Dullahani are remarkably durable, to the point a duller weapon can't even penetrate the skin. They stand at roughly 6 feet (1.8 meters) on average, with the average human being 5'5 (1.6 meters) and other races being usually shorter than even that. The head and body are connected via a sort of 'portal' that links things like blood vessals, nerves, etc. A Dullahani can detach their own heads and reattach them at will, but a decent hit (450 pounds of force) can knock them off unwillingly. A Dullahan retains control over the body while the head is detached.
When the head is removed from the body, the neck becomes a highly vulnerable spot. A decent stab into the exposed neck would cancel the connection between the head and the body, essentially turning the Dullahan into a decapitated human. They have access to 15th and 16th century technology for the most part. Magic isn't really a battlefield 'weapon' as in most cases its a long, complicated ritual to get some basic results.
Dullahan on wikipedia, for those who haven't heard of one before: https://en.wikipedia.org/wiki/Dullahan
EDIT: I was asking about what weapons to use against them, and which armors they should use. I didn't think I should seperate them into seperate questions.