I know someone aged 76 who has been told in April 2003 that she has
lung cancer. About a 5 mm mass and she is in a lot of pain despite
taking very strong pain killers. She has just stated on RADIATION
TREATMENT.
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(1) What are the benefits of this type of treatment, if so what?
(2) Is there a possibility/likehood of the pain lessening?
(3) Is there is any possible side efefcts if so what can they be?
(4) What useful beneficial tips can the person bear in mind apart from
trying to maintain a benefical and positive outlook!

Probably you mean 5 cm, which is around 2 inches. If it's causing
severe pain, it's presumably because it's growing into the tissues
next to the lung. That means it's not likely curable. The purpose of
the radiation is to shrink it and to lessen the pain: so decreased
pain is probably the primary goal of the treatment, and is fairly
likely to occur. Side effects can include tiredness almost certainly,
lung irritation causing cough; rarely enough damage to cause bleeding,
etc. But under the circumstances, it's likely the best and only
alternative.