Celtic Reconstructionist Paganism (also Celtic Reconstructionism or CR) is a feckin' polytheistic, animistic, religious and cultural movement, like. It is an effort to reconstruct and revive, in a holy modern Celtic cultural context, pre-Christian Celtic religions.
Celtic Reconstructionist Paganism originated in discussions among amateur scholars and Neopagans in the oul' mid 1980s, and evolved into an independent tradition by the bleedin' early 1990s. Whisht now. Celtic Reconstructionism represents a bleedin' polytheistic reconstructionist approach to Celtic Neopaganism, emphasisin' historical accuracy over eclecticism such as is found in many forms of Neo-druidism. Story? Currently, "Celtic Reconstructionist Paganism" (CR) is an umbrella term, with a feckin' number of recognized sub-traditions or denominations. Whisht now and eist liom. 
How about a bit more, for good measure. Grand so:
While the bleedin' ancient Celtic religions were largely subsumed by Christianity, many religious traditions have survived in the feckin' form of folklore, mythology, songs, and prayers. Me head is hurtin' with all this raidin'.  Many folkloric practices never completely died out, and some Celtic Reconstructionists (CRs) claim to have survivals of Irish, Scottish or Welsh folkloric customs in their families of origin. Jaykers! 
Language study and preservation, and participation in other cultural activities such as Celtic music, dance and martial arts forms, are seen as a bleedin' core part of the tradition. Participation in the feckin' livin' Celtic cultures - the oul' cultures that exist in the bleedin' "areas in which Celtic languages are actually spoken and in which Celtic traditions have been most faithfully handed down to the bleedin' present day" - is a vital part of their cultural work and spiritual practice, that's fierce now what?  The protection of Celtic archaeological and sacred sites is important to Celtic Reconstructionists. G'wan now and listen to this wan.  When construction of the feckin' N3 motorway in Ireland threatened to destroy archaeological sites around the Hill of Tara, Celtic Reconstructionists (among others) organized protests and a coordinated ritual of protection, would ye believe it?