It does indeed sound as though your sister-in-law is showing signs of mania and I'm glad that you're trying to get her some help.
One way to get help for someone who is reluctant is to frame it as a family intervention- saying that you are all having some problems and could use some help to communicate better with each other. Then schedule an appointment with a family therapist, go together as a family, and talk about your concerns for your sister-in-law once you're there. My guess is that there ARE in fact ways that your sister-in-law's behavior is affecting and concerning the whole family, so this kind of meeting could really be useful to all of you.
Another way to get some help for your sister-in-law is to take her to a local hospital or crisis center. If she meets criteria for dangerousness to self or others (and suicidal or homicial feelings) then she can be involuntarily sent to a psychiatric facility where she will be further evaluated and then either committed or released.
The tricky part is when someone doesn't meet criteria for involuntary commitment, won't agree to family or individual therapy, and continues to display manic-type behaviors. I don't know how the mental health system works in Ireland, but in the US there is no good way to get help for someone who fits this category.