Sure. What was life like in medieval Ireland? Starting with your cultural setting, was there a government? Who ran it? A military? Religion? How was the typical home constructed? What sorts of occupations did people have?

As for definitions:

What were government officials called? Where did they meet? What were the meetings called? Are there documents that define law? Who enforces the law? Are there courts? Were average citizens afraid of the government, laws, enforcers, courts?

Military leaders, units, soldiers? What weapons and armor did they use? What did they call the place where they they practiced?

Were there religious leaders? What were they called? Did they perform ceremonies? What were those called? Did they use any sort of symbolic objects?

What were homes called? Professions? What did people eat? What did they eat it out of? Were there hospitals? Doctors? What were those called?

Etc. I would just keep brainstorming the culture, research answers you don't know, start a list of things the average person wouldn't know and define / explain them. Spend 45 minutes on it and give yourself credit for research, cultural setting, and definitions list.

For example, I found this site via a quick search (therefore I can't speak to the accuracy or authenticity):

http://www.yourirish.com/history/medieval

Here's a passage:
Life in Ireland wasn’t perfect as there had been internal feuds with the clans who had control over different areas. Ireland had been split into several large Kingdoms with lesser Kingdoms and Dynasties being established so there would always be some sort of dispute of who owned what land.

If this affects your story in any way, you might have the following definitions on your list:

* Clan
* Kingdom
* Dynasty

Cheers,
Michelle