Can't attest to Scotland, though Iron Age burial custom in the north of Ireland - which was culturally very similar to Scotland, with migration and religious ideologies moving in a two-way process - was for cremation, and there was probably some excarnation as well.

Druidism was popular in Ireland, if the early Christian monk's writing is anything to go by. Votive offerings were made, possibly to propitiate the gods and ensure the continued fertility of the soil. Druids would have worked as intermediaries between the spirit world and reality, and peasants would have been terrified of offending the gods.

There was certainly a belief in life after death, and if the stone circles and henges are anything to go, sun worship was a part of the cultural package.

Irish culture became more ingrained in Scotland as time went on, and we really don't know much about the Picts, but if you believe that they were a Celtic society - which is prevailing theory - then they would have had gods similar to Dagda, Morrigan, and the 'Dis Pater' that the Romans claimed the Gauls worshipped.

As the Iron Age progressed, and the influence of Roman and Romano-British settlement was felt, inhumation became more popular.

Good sources on this would incude Colin Renfrew and Barry Raftery, whose Pagan Celtic Ireland I've drawn on.