Am I allowed work for both Companys?

I am employed by Cable company but classed as a Private sub-contractor, company provides Me with transportation ang equipment for the cable installaion.
I have now been offered a Job with another company installing "Broadband"Am I allowed work for both Companys
, And who do I have to Declare it to. Rep of Ireland??

If you are truly self-employed then there should be no problem doing work for both companies, assuming you have the time to complete all the work. However, you may find that this is when the first company informs you that you can only work for them and if that is the case, you were never, truly, self employed.

In the UK, the fact that the company provides you with transport and materials would give weight to the argument that you are an employee. If the company also tells you when to do the work and forbids you from sending another person in your place to do the work, that would also indicate that you are an employee. Are you paid for holidays or sickness? Does the company provide public liability insurance? These are more indicators.

If you are presently self employed, who did you declare this to? How do you pay your tax? It would be the same for a second self employment.

Not sure. In England you would be classed as self-employed and you would need to tell the Inland Revenue. Under self-employment you would be entitled to work for whoever you liked, but would be responsible for paying own tax and NI.
If you are classed as employed then you can still work for more than one company, but you will be taxed at a higher rate on one of the jobs if your earnings/hours took you over a certain limit.
You need to check what the small print of any contract for the deal says. They often have details about the companies take on "other" employment, particularly if it is with a rival firm. And if by declare you mean to the inland revunue, yes you must otherwise you'll get in a world of poo, fines, bans or even a prison sentance. It really is NOT worth risking!