Guys,

How come in the North they say 'oo' for a lot of words that are spelt like 'dalladh' for example. Like 'dalloo'

Is it a change that happened or is it something to do with the older spellings or something?

The suffix -adh is pronounced pretty different in the dialects (and even different in different circumstances)
The "original" pronunciation /að/ couldn't survive because of loss of /ð/ sound.
So, either
- change of consonant /əð/ -> /əɣ/ -> /əx/ (or -> /əv/, /əf/ or /əg/) or /əð/ -> /ət´/ (rather /əṭ/)
- total loss of consonant /ə/ or
- some kind of vocalization /əð/ -> /əɣ/ -> /əv/ -> /u:/.
had to happen (and all of them happened).

/u:/ is used in Ulster in nouns, verbal nouns and verb forms,
e.g. tuigfeadh Pól /tik´u: Po:l/ (Paul would understand)
except in verb forms before the pronouns sé, sí, siad:
e.g. tuigfeadh sé /tik´əṭ s´e/, he would understand

In South Connacht, /u:/ is used in autonomous past tense verb forms only, e.g.
tuigeadh é /tig´u: e/ (he was understood)

In Munster, /u:/ isn't used at all, afaik.

In Munster it’s /əx/ in verbal forms (past habitual do chuireadh /də xirʹəx/ ‘used to put’, conditional do ghlanfadh /də ɣlɑnhəx/ ‘would clean’, etc.) and /ə/ in verbal nouns (pógadh /poːgə/ ‘kissing’). Haven’t seen /uː/ for -adh in any Munster materials.

Corkirish gives also /əg/ for past autonomous, eg. do glanadh /də glɑnəg/ ‘one cleaned’ (A. Doyle’s An Ghaeilge gives /əx/ here too – perhaps Cork vs Kerry difference?).

Yes, past autonomous in Dingle /əx/, Muskerry and Ring /əg/, Cléire /əv/

I know a family in Dún Aill Co Waterford, Ó Murchadha, who also use the spelling Ó Murchú. This may be from influence of the standard and may not reflect their usage of the language in all aspects.