Samhain is not the "Celtic New Year". It is not even reliably considered "the" Irish New Year to the exclusion of all other systems of reckoning the year. It might have been "a" new year, one system among several, though the evidence for this is sketchy. Nevertheless in 19th century Ireland it was considered an extremely hazardous and important time.

Here is a remark by Ronald Hutton on Caitlín Matthews:

"Most of the approach adopted by Ms. Matthews can be found in scholarly books upon the Celts produced before the 1970s, when the new care in textual criticism and mutual appraisal came to be adopted in universities. But her work is also conditioned to the needs of her audience. To a great extent, the world of the modern 'Celtic mysteries', like that of the 'earth mysteries', is self-contained. Thus, her bibliographies are filled on the one hand with old academic works and on the other with writings of the contemporary Celtic mystical movement right up to 1990. What are missing are the scholarly publications of the 1980s, many of which have radically altered existing views of the sources for our knowledge of the ancient Celts. But then any 'movement', or 'tradition', tends to build upon iteslf, and is not much given to questioning its basic texts. Caitlín Matthews and her colleagues are not really concerned with the past, so much as with the present and the future. They are creating a mysitcal tradition out of old materials but suited to modern needs."
--Ronald Hutton, The Pagan Religions of the Ancient British Isles: Their Nature and Legacy, Blackwell, Oxford, 1991 (Paperback reprint 1993), p. 144.

T.