Since 2008, I have had 3 pensions from Europe and a UK company pension. HMRC use the UK company pension, via my tax code, to pay tax on the PAYE system for the non-UK pensions (there are now others including the UK State pension and US SS).
All non-UK pensions are combined according to country of origin on the Foreign Pages (SA106, F 2) under Overseas pensions, social security benefits, etc., and the total declared in Box 9. None are IRAs, so I have no knowledge of how an IRA is handled, but I would warn KISS! Unless there are special HMRC considerations or the DTA is involved, KISS!
I KISS! So far, I've had absolutely no problems with HMRC regards how I report.
I do have other problems which Nan D. and durhamlad will not have. The constantly changing exchange rates make it impossible for the PAYE amount to be accurate (or even reasonably accurate). Therefore, I have PAYE and, depending on the exchange rates, additional or pre payments in January and July. If any one else has tax on foreign pensions paid via their tax code, I would issue a warning - I've just received my Tax Code for 2018/19. It does not take into consideration the now 100% taxation of foreign pensions. I'll issue one further warning. When I received my statement from the tax inspector (the normal letter after using the paper reporting method) for 2016/17 (due 01 Jan 2018) the tax on interest payments for bank/building societies was missing (multiple £thousands) even though the amounts of interest had been included properly on my filing (confirmed when I called to correct HMRCs error). BE CAREFULL OUT THERE, and check all correspondence carefully.