I hadn't noticed the question words, though obviously had noticed the loan words from garçon, eglise, and in some dialects table (which is also a loan word in English). You go to a place like Cove on the South coast and you will hear a lot more. Some of them are from the shared from the common influence of Greek and Latin, some direct commerce with the French, some from the fact that there are still words in French that retain traces of the pre-Roman Gallic language.
There was no Roman invasion of Ireland! However, the language of Christianity when it came to Ireland (and of the literacy practised by Christian monks) was Latin. Hence Nollaig (and French Noël) from Latin natalacia (birthday celebration), as well as such words as eaglais (church) < ecclesia, leabhar (book) < liber, peann (pen) < penna (lit. feather), sagart (priest) < sacerdos, scríobh (write) < scribo.
Note, though, that there are some resemblances which derive from loans that went the other way -- with Latin borrowing from Celtic. An instance of this is Latin caballus, which was a Roman borrowing from Gaulish; Irish capall and Welsh cefyll come directly from the same Celtic root.
At the beginning, in the same position as Is.
Ní maith liom an Nollaig.
Is aoibhinn liom ... Aoibhinn means delightful, blissful.
Is breá liom ... when you like something a lot.
You don't. English uses "do" as an auxiliary verb in questions ("do you like...?", "does he eat...?" etc) and in replies to questions "I do", "he does", including answers to questions like "who likes...?"
Irish doesn't use an auxiliary verb like "do", so you can't just ask "Who doesn't?", you would have to specify the verb in question - "who doesn't like?" or "who doesn't eat?" etc.
It's not slang - as hilarymcca5 explained it's an abbreviation using an approximation of a Greek character that stands for Christ. Though I suppose that many of the people using it have no idea about that history, and some of them may consider it slang.
Xmas, and variants such as Xtemass, originated as handwriting abbreviations for the typical pronunciation /ˈkrɪsməs/. The "X" comes from the Greek letter Chi, which is the first letter of the Greek word Christós (Χριστός), which became Christ in English.
As an abbreviation, it's relatively uncommon in Ireland - it wouldn't be an automatic choice for the contributors to include as a translation of Nollaig.