Can i reach all users from one specific country in Facebook?

Yes, you can target Facebook ads by location: https://www.facebook.com/business/a/online-sales/ad-targeting-details

This setting is in the ad manager. If you're looking for a certain nationalistic feel you should also target people that are politically active. Facebook offers this option as well for certain countries.

Technically yes, you can. But would always advise a more targeted approach to advertising on Facebook, or any marketing channel. Although you product may be nationalistic, just because someone is Irish doesn't mean they would have an interest in buying it. You'd have to be a certain type of person. Would strongly advise more thought around the audience you are trying to reach and what motivates them along with thinking about your current customers to work out a customer profile. Then developing a highly targeted ad campaign around that with creative and copy that will resonate with your potential customer. Hope this helps, let me know if you'd like to talk more.

Yes, it is possible to reach all the users in a specific country using Facebook if you follow the following procedure:
Add or Edit Country Restrictions: You can choose to show or hide your Page only to people in some countries. To add or edit country restrictions for your Page, you'll need to be an admin. If you are an admin:
1. From your News Feed, click ‘Flag icon’ Pages in the left menu.
2. Go to your Page and click ‘Cog and wheels’ Page Settings in the bottom left.
3. From General, click Country Restrictions.
4. Add one or more countries and choose to hide or show your Page to viewers in those countries.
5. Click Save Changes.
If no countries are listed, your Page will be visible to everyone.
Besides if you do have any questions give me a call: https://clarity.fm/joy-brotonath