Couple of interesting points from my application that may help others...
Once posted in GB - it took over a week (10 days) to leave the UK - this must be a Brexit issue as it was nowhere near this long before - I have seen others say the same on here and the FBR chat. I also had to submit a customs declaration with the postage documents - good to see that red tape getting cut
Back to the application itself, it took just over a week to verify documents once received in ROI and then around 4 weeks of 'processing.' They called my witness 4 days ago and it immediately changed to printing once confirmed. It seems that calling the witness is the final step before printing and being issued a passport.
I submitted an original driving licence and certified copy of passport - both fine. Also submitted 4 proof's of address/name and followed the advice for Northern Ireland as we have the same public services in GB (submitted one of each document they asked for, technically two more than they needed, i.e. HMRC, 2 x NHS, Council Tax etc)
Passport in the post to me 1 month, 1 week and 4 days post application, with a 10 day hold up in the UK. Not too shabby at all after a 29 month wait for citizenship!