What did Marks Models say when you contacted them? Do they have a lead time for arrival in stock? If so, what is it?
Hatton's and Rails deliver to Ireland. Or are you looking for an exclusively Irish alternative to Marks?
Brian Collins doesn't stock track, as far as I know, but he might be worth contacting and asking.
https://briancollinsenterprises.ie/
RB Models only have concrete sleeper track listed. Again, might be worth contacting them to see if they can order anything and if the lead time is better than Marks.
https://rbmodels.ie/product-category/trains/train-tracks/peco/
Hatton's and Rails are both very good for sales and customer service, prompt and concise.