"Tonight" is a title given to a song that U2 had possibly worked on in the April 1978 demo sessions at Keystone Studios. We believe the proper title for the song was "Night Fright", and that "Tonight" (and "Live My Life Tonight") are names given to it on bootleg recordings. There are references to it that suggest that "Night Fright" is likely the title Bono intended.
We have taken an in depth look at U2's pre-Island Records demo sessions in two parts. Part One dealt with 1978 - February 1979. Part Two dealt with the latter part of 1979 until early 1980. "Tonight" is discussed in Part One. The song is also looked at in more detail under the entry for "Night Fright" here.