Unsure about how many gaps to fill, affordability, tax etc

Many thanks to all the contributors on this brilliant forum.
My wife has applied (having trained and worked as a nurse in Manchester) and has received a letter advising she has 7 qualifying years of NIC's up to April 2024.
The letter confirms she is class 2 and they have provided the shortfall table per year. She thought she only had 4 but didn't know about the bonus 3 added automatically.
There is a deadline of April 5th this year to buy back as far as 2006/2007 at circa £160 per year (total of 13 years)
However she is reluctant to fork out the 2500 or so now given other financial commitments but might look at filling say 3 years now and then paying annually going forward until aged 66 (11 years time). Also a bit concerned about waiting until 66 or 67 to reap the benefits (health permitting)
What are people's thoughts on this as an affordable solution for us at this juncture ?
She does have the opportunity to clock up the 35 years which will obviously be lost post April 5th 2025 as will the opportunity to go back 10 years.
Advice welcome given the affordability aspect please.
Does it matter which years to fill - I presume the older ones makes sense.
What are the tax implications here in Ireland - how much tax is payable on a uk pension ?

Difficult to advise on affordability, as that's your personal situation. But best advice is to buy whatever you can afford.

For example, I've been putting aside a few quid a month for this for the past few years. By the time 5 April comes I'll have paid up to the 2020-2021 tax year, and will pay the remaining 4 outstanding years by the end of 2025 to bring me up to date with 27 years in total. This leaves me with 8 years to buy over the next 19 years before I retire.

Nothing wrong though with buying the minimum amount now that sees you buying another year every year until retirement. Only risk is if the Class 2 gravy train is ended. But all approaches come with some risk.

RE tax - it is treated like any other income in retirement. View it as a personal pension.

Your wife has 8 years now, and needs min 10 for any UK pension.
She has the opportunity to buy 13 back years, plus 11 forward, (Max possible pension = 32/35th)
BUT the 06/07 - 18/19 opportunity will be lost after this April
While 24/25 will be available until 2031, 25/26 until 2032 etc (unless they change the 6 year rule)

It may be an idea to forget about 24/25 for now, and buy back at least 2 (to get to 10 years) plus as many of the early years as affordable at this point in time. The 52 week years are slightly cheaper than 08/09 and 14/15
Best of Luck!