This paper addresses an important topic: surface ozone (O₃) trends across Ireland, using long-term observational data and model (CAM4‑Chem + tagging via TOAST 1.0) source-attribution analyses. The combination of measurements, trajectory / sector classification, and modelling is commendable. The regional focus on Ireland particularly the background coastal site at Mace Head adds value, since many O₃ studies focus on larger continental areas. The methods are reasonably described, and the results are of interest for air‐quality / atmospheric chemistry / policy communities. That said, the manuscript requires substantial improvement in several areas before it is ready for publication: grammar & sentence structure need polishing, some methodological choices need clarification, some results could be better contextualized, and a number of references appear missing or inconsistent.

Major concerns

• The abstract and introduction present the study aims, but the phrasing is sometimes confusing. For example: “Using innovative trajectory analysis, O₃ concentrations, exceedances and were identified by sectors…” (L17–19) – the sentence is awkward, and a verb seems missing (“and … were identified”).
• It would help to explicitly state the hypotheses (e.g., “We hypothesise that urban sites will show increasing O₃ due to reduced NOₓ titration while background coastal sites will show declining O₃ thanks to precursor reductions”). A clear statement of hypotheses will strengthen the framing.
• The authors mention “innovative trajectory analysis” but more precisely explain what is new compared to previous work. Many previous studies have done back-trajectory classification. Clarify what is novel.
• I suggest a thorough pass with a native English speaker or professional editing service to improve readability, grammar, and logical flow.
• The trajectory classification: The criteria (72h over ocean) for “clean sector” should be justified more clearly. Why 72 h? Why 100 m height at 6:00 UTC? Are results sensitive to these thresholds?
• The modelling: The CAM4-Chem grid resolution is coarse (1.9° × 2.5°) (L229). This is acknowledged by the authors in discussing biases (L486–490). However the implications for interpretation (especially for urban sites) should be more emphasised. Is the coarse resolution adequate for urban site comparisons?
• The source tagging via TOAST is described, but I would recommend including a validation of the tagging method (or refer to validation studies) in a little more detail. For instance, what is the error/uncertainty associated with the tagging?
• Statistical trend methods: The authors use Theil-Sen slopes + Mann-Kendall tests (L180–185). It may help to compare with alternative methods or at least to discuss the limitations (e.g., non-stationarity, autocorrelation).
• In modelling vs observations comparison (Section 3.4.1), only five sites are used. Why these? Are they representative? Could more sites be used or reasons given for the selection?
• The result that urban sites show increasing O₃ trends (e.g., Rathmines +0.27 µg m−³ yr−¹ for full period) (L363–365) is interesting. But the mechanism is briefly mentioned (“weekend effect”, NOₓ titration) (L311–313); this could be developed further, perhaps linking to local emission inventories or changes in VOC/NOₓ ratios over time in Ireland.
• The discussion of the 2020 COVID-19 lockdown (Section 3.3) is interesting (L451–458). But the data are limited (March-May 2020) and a more nuanced discussion of meteorology confounding effects would strengthen the claim.
• The authors report a decline in spring-time O₃ at Mace Head but an increase in winter trends (L465–470). This is a key finding. However, the discussion linking these to emission reductions, stratospheric intrusion, hemispheric transport is somewhat speculative and could be better supported by citations or sensitivity tests.
• It would help to compare the Irish trends with trends elsewhere (Western Europe, North Atlantic) more thoroughly — e.g., are the trends consistent with broader European background ozone literature?
• The uncertainties in modelling and measurement (especially for attribution of sources) should be more explicitly discussed.

Minor Comments:

• Line 229 - It would be useful to maintain the same order of stations throughout all tables and figures.
• Line 326 - Figure 5 Why these NO2 and CH4 sites are selected.
• Line 342 - This sentence needs to be rewritten for clarity; the current wording is difficult to follow.
• Line 353 Include details of the model grid used over Ireland to support the interpretation of spatial results.
• Line 415 Figure 8 Clarify what the SHIP parameter represents. It is not defined in Table 3.
• Line 456 - Please provide details of the method used to define background and EU- influenced airmasses. This information is essential.
• Line 543 Check this sentence , it appears incomplete and needs revision.
• Line 551 Figure 13: Change the colour scheme to improve readability.
• Supplementary material - Section 3 is missing. Please correct the numbering and ensure figures.