From reading your last post, you say you might use the UK property as collateral. To do so you will have to divulge that you own the property.
What if you do not declare the property but the bank then says "if only you had property that could be used as collateral we would be happy to give you the mortgage". You can't really change your mind and go "oh by they way..
This of course is in addition to the whole issue of lying on a mortgage application which is basically what you will be doing. You will need to sign a declaration stating that all facts in the application form are true (if you do not declare ownership of another property then that declaration will be false)
Finally banks have ways and means, if there are any transactions in your Irish current account statements linking you to the UK property in any way then they will question these.
Be honest up front, your salaries are strong enough to support both mortgages and this is what the banks will be looking at. If the bank wishes to use the house as collateral then you can always insist that this is not part of the deal.
Your friend may be pretty but I would question their financial savviness.