Constrained Poisson model / Bayesian Poisson model

Hi all,

I am dealing with a problem about my linear Poisson regression model  
(link function=identity).

I am using the glm()-function which results in negative coefficients,  
but a negative influence of the regressors wouldn't make sense.

(i) Is there a possibility to set constraints on the regression  
parameters in glm() such that all coefficients are positive? Or is  
there another function in R for which this is possible?

(ii) Is there a Bayesian version of the glm()-function where I can  
specify the prior distribution for my regression parameters? (e.g. a  
Dirichlet prior s.t. the parameters are positive)

All this with respect to the linear Poisson model...

Thanks in advance!
Best,
Mara