"It is a menu."
Translation:Is biachlár é.
Thanks so much! I find this a little confusing still, but now that I know the basic difference I can start to be on the lookout for this mistake. I have been interested in learning Irish for about 20 years now but only recently with more internet presence do I feel like I have a shot at making some good headway. I am so glad that Duolingo have decided to tackle this; I was one of the many asking for it, as I have valued the commenters' input very much in starting to learn French. For others who may be confused, I found this writeup resource with exercises (free wikibooks article) http://en.wikibooks.org/wiki/Irish/Basic_Sentence_Structure
This is a common problem for learners and people struggle with it even until their final year of school. The copula (an chopail) 'is ...' is used when saying a noun is another noun. It is used to describe a permanent identify of something.
In 'it is a menu', the nouns are 'it' and 'menu'. You could use 'tá sé' if describing the noun with an adjective. For example, 'tá an biachlár nua' ('the menu is new'). You can also use adjectives in the copula, but the 'two nouns' rule still applies: 'is é an biachlár nua' ('it is the new menu').
If you use the copula with only one noun and one adjective, you imbue the noun with that quality. 'Is maith é' means 'he is a good person', where as 'tá sé go maith' means 'he is well'. The difference is that the former is supposedly a permanent, intrinsic quality, while the latter is potentially temporary.
Have a look here for a quick description: https://en.wikipedia.org/wiki/Irish_grammar#Syntax
There are other websites which describe an chopail in great detail.